THAT IS ONE LONG ECLIPSE!
-----------------------
John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskie.org
2008 November 15 initial
2009 September 1 current
Introduction
----------
Epsilon Aurigae (EPP-sih-lonn au-REE-gigh) is a 3rd magnitude star
in the 'Kids' asterism of Auriga (au-REE-ga). It, with eta and zeta,
configure small goats nestled in the left arm of the Charioteer. They
were from time to time over the ages separated into their own
constellation Haedi.
Epsilon is the star at the acute point of the triangle epsilon-
eta-zeta. Or it is the star of that triangle nearest to Capella. It
has the proper name Almaak, nowadays rarely used by astronomers, but
it shows that the star was known well enough to earn a name.
Epsilon's fame comes from its variations of light and other
radiation thru eclipses. It is an eclipsing binary, somewhat like
Algol in Perseus. Such eclipsing stars go thru their cycles in days or
weeks. However, unlike Algol and other familiar eclipsing stars,
epsilon has a period of, uh, 27.1 YEARS. When the eclipse comes, it
lasts about 20 MONTHS.
The last eclipse was in 1982-1984. The next is in 2009-2011.
Because of the rarity of the eclipses and the potential importance of
this star, preparations for eclipses begins a year and more ahead. For
home astronomers, we will witness an event that likely is a once-in-a-
career occasion. Few astronomers stay in the profession long enough to
enjoy two rounds of epsilon Aurigae.
Visibility
--------
Epsilon is far enough north in declination that from mid north
latitudes it is almost circumpolar. More practicly it is visible at
some hour, if an inconvenient one, on every night of the year. Here is
a table of rising, transit, setting during a year at 10-day intervals
for New York City. All hours are EST, no EDST used here.
+--------+-------+-------+-------+
| | rise | trans | set |
| date |azm 23|alt 87|azm 337|
+--------+-------+-------+-------+
| 01 Jan | 12:26 | 22:13 | 08:03 |
| 11 | 11:47 | 21:33 | 07:24 |
| 21 | 11:08 | 20:54 | 06:45 |
| 31 | 10:28 | 20:15 | 06:06 |
| 10 Feb | 09:49 | 19:35 | 05:26 |
| 20 | 09:10 | 18:56 | 04:47 |
| 01 Mar | 08:30 | 18:17 | 04:08 |
| 11 | 07:51 | 17:37 | 03:28 |
| 21 | 07:12 | 16:58 | 02:49 |
| 31 | 06:32 | 16:19 | 02:10 |
| 10 Apr | 05:53 | 15:40 | 01:31 |
| 20 | 05:14 | 15:00 | 00:51 |
| 30 | 04:34 | 14:21 | 00:12 |
| 10 May | 03:55 | 13:42 | 23:29 |
| 20 | 03:16 | 13:02 | 22:49 |
| 30 | 02:36 | 12:23 | 22:10 |
| 09 Jun | 01:57 | 11:44 | 21:31 |
| 19 | 01:18 | 11:04 | 20:51 |
| 29 | 00:39 | 10:25 | 20:12 |
| 09 Jul | 23:55 | 09:46 | 19:33 |
| 19 | 23:16 | 09:06 | 18:53 |
| 29 | 22:37 | 08:27 | 18:14 |
| 08 Aug | 21:57 | 07:48 | 17:35 |
| 18 | 21:18 | 07:08 | 16:55 |
| 28 | 20:39 | 06:29 | 16:16 |
| 07 Sep | 19:59 | 05:50 | 15:37 |
| 17 | 19:20 | 05:11 | 14:57 |
| 27 | 18:41 | 04:31 | 14:18 |
| 07 Oct | 18:01 | 03:52 | 13:39 |
| 17 | 17:22 | 03:13 | 12:59 |
| 27 | 16:43 | 02:33 | 12:20 |
| 06 Nov | 16:03 | 01:54 | 11:41 |
| 16 | 15:24 | 01:15 | 11:01 |
| 26 | 14:45 | 00:35 | 10:22 |
| 06 Dec | 14:05 | 23:52 | 09:43 |
| 16 | 13:26 | 23:13 | 09:03 |
| 26 | 12:47 | 22:33 | 08:24 |
+--------+-------+-------+-------+
It is best to observe while epsilon is above the horizon mists,
between about 1-1/2 hour after rise and 1-1/2 hour before set. When
epsilon is dimmed by low altitude, nearby Capella leads you to it.
The star
------
The star has many catalog designations. A few are:
-------------+------------
catalog | designation
-------------+------------
proper name | Al Maaz
Bayer | epsilon Aurigae
Flamsteed | 7 Aurigae
BSC, HR | HR 1605
Smithsonian | SAO 39955
PPM | PPM 47627
Henry Draper | HD 31964
Tycho | TYC 2907 1275 1
Hipparchos | HIP 23416
Bonner Durch | BD +43 1166
Aitkens | ADS 3605
Washington | WDS 05020+4349
-------------+---------------
The name Al Maaz, or plain Maaz, is hardly used among astronomers,
even those versed in star history.
The Aitkens and Washington catalogs are for double stars, being
that epsilon is n eclipsing binary.
Some statistics of the star are:
------------------+--------------
right ascension | 05h 01m 58.1s
declination | +43d 49m 24s
galactic long | 162.7 deg
galactic lat | +1.08 deg
app magn | +3.03 at maximum
distance | 2,038 ly
spectrum | F0-I
color index | +0.53
abs magn | -5.95
luminosity | 20,690 Sun
Sun's magn | +13.8
mass | 20 Sun
radius | 120 Sun, ~1/2 AU
temperature | 6,400 K
peak wavelength | 453 nm
radial velocity | -1.4 Km/s
tangent velocity | 6.9 Km/s
proper motion | 0.00232 sec/y
translation | 1.478 AU/y
------------------+-----------
All of these values relate only to the primary star of the pair.
The secondary is so poorly understood that few firm specs are known
for it.
History
-----
Epsilon was known for at least two thousand years, being noted by
Ptolemaeus in his Alamgest. It was the epsilon star in Bayer's
Uranometria and the 7 star of Flamsteed. There is no positive evidence
that the star's dimmings in eclipse were noticed before the 1700s.
There is suggestion that a fainter magnitude rank assigned to the star
in some ancient tables is a real measure of the star, but the number
could just as easily be a miscopying, mixed in with other genuine ones
I do not here give a detailed chronicle of the star, there being
excellent historical accounts in the litterature. Here is only a
summary of previous eclipses.
It took a while to recognize epsilon as a variable star because it
remains at full brightness for over two decades. This fooled many
astronomers to treat reports of dimming as erroneous. An other cause
for late recognition was that until the mid 1800s the study of
variable stars was too crude. Only a few peculiar stars were known to
alter their brightness and all of these did so in days or weeks. They
merited only a paragraph or two in textbooks of the time.
The first definite awareness of epsilon's dimming was during the
eclipse of 1821-1824. The star was a sensation then for the prolonged
decline and interminable dwell at minimum light. But with there being
nothing of a modern theory of stars, the apparition remained a
wonderful curiosity of the sky. After returning to full brightness and
not doing anything odd for a decade, the star was generally forgotten.
The eclipse of 1846-1848 was well observed by the fathers of
variable star astronomy and a good pattern was obtained for its
changes of light. Periodicity was not yet discovered. The 1874-1875
eclipse was also well watched, now with visual spectroscopy. The
repetitive cycle was still not discerned, there being only now two
good minima plus scattered reports from earlier ones.
In the late 1800s the nature of eclipsing binaries was worked out,
solving the enigma of Algol and Sheliak (beta Lyrae). By historical
note, Algol was explained in the mid 1700s as an eclipse of the star
by a planet! This, by Michell, was the first serious notion that other
stars could have planets around them. Of course, Algol's companion by
spectroscopy proved to be an other, dimmer, star.
After the round of 1900-1902, with three good eclipses on record,
the eclipsing binary solution was issued. There after, studies of the
star thru the 1928-1930, 1955-1957, and 1982-1984 eclipses confirmed a
general circulation of a darker star around the obvious brighter
component.
The 1955-1957 and 1982-1984 rounds were coordinated by a specific
campaign of observation. The others attracted attention of a
substantial number of individual astronomers with only casual
interaction among them. The 1980s watch attracted home astronomers in
large numbers. A much wider turnout of astronomers is expected for the
next round as part of the International Year of Astronomy.
I remember observing the eclipse of 1982-1984 quite well, my first
round for studying this star. I followed news from the 1965-67 eclipse
but only occasionally looked at the star.
Trouble with the simple solution came in the 1930s from the
spectrum and excedingly long duration of the eclipses. How could a
binary star have so long a crossing of its components? Why doesn't the
second star show up in the spectrum? By the late 20th century we
figured out that the secondary star is quite invisible in optical
wavelengths, registering nothing in the spectrum. Mind well that
before the 1980s there were only ground-based observatories that could
not detect radiation much beyond what the air passed thru as light.
Space observations for the 1982-1984 eclipse and those from new
high-elevation telescopes showed that the companion does radiate
weakly in mid and far infrared, but not in visible wavebands.
The simplest, at first, solution for the long duration of the
eclipse was that the companion is extremely large, humongous to the
max, and was in a long-period orbit. Various models proposed a dark
ultra-super giant star that orbited dozens of AU from the main star.
It would be about the very largest star in the heavens!
The campaign to observer the 2009-2011 round is underway since
early 2008. Preparations begin at observatories in 2006. With the
International Year of Astronomy coming in 2009, epsilon Aurigae became
a centerpiece target of public outreach. The hope is that for the
first time in recorded history, useful observations could be collected
from millions of layfolk.
Zeta Aurigae
----------
Discussion of epsilon often include a neighbor star in the Kids,
zeta. This is the star farthest from Capella, requiring a line from
there to cross thru the triangular asterism.
The prime caution is that you must NOT bank a measurement of
epsilon off of zeta during spring of 2009. It, too, is a variable
star! It's also an eclipsing binary of quite 1/10 the period of
epsilon, about 2.7 years. Its next eclipse is in March of 2009 and
lasts 35 days. After its eclipse, zeta returns to full brightness and
is a stable standard to assess epsilon against. Its second next
eclipse is after epsilon finishes its own.
If you know other home astronomers new to the epsilon Aurigae
story, be SURE to caution them about zeta for the 2008-2009 apparition
of epsilon. Deliberately ask if they called on zeta as part of their
epsilon's measurement process. If so, it may be kindest to discard
that measure or reconstruct it from other stars in the report.
On the other hand, zeta can be an excellent star to examine for
those too impatient with epsilon's languid pace of action. Zeta's
cycle completes within spring of 2009 in convenient evening hours.
Light variations
--------------
Epsilon Aurigae exhibits more or less the same lightcurve as a
classical eclipsing binary, only taking a hell of lot longer to go
thru its cycle. Here's a comparison of several cycles:
+-------------------+-----------+------- ------+
| phase | 1955-1957 | 1982-1984 | 2009-2011 |
+-------------------+-----------+-----------+-----------+
| partial begins | Jun 1955 | Jul 1982 | Aug 2009 |
| partial duration | 135 day | 142 day | 137 day |
| totality begins | Oct 1955 | Nov 1982 | Dec 2009 |
| totality middle | May 1956 | Jun 1983 | Aug 2010 |
| totality duration | 394 day | 447 day | 446 day |
| totality ends | Nov 1956 | Feb 1984 | Mar 2011 |
| partial duration | 141 day | 65 day | 64 day |
| partiality ends | Apr 1957 | Apr 1984 | May 2011 |
+-------------------+-----------+-----------+-----------+
| eclipse duration | 670 day | 654 day | 647 day |
| time since prev | 9,885 day | 9,863 day | 9,885 day |
| amplitude | 0.75 magn | 0.91 magn | 0.70 magn |
+-------------------+-----------+-----------+-----------+
Dates for 2009-2011 are deliberately rounded to the month to
recognize that they are predictions and can only be approximate. The
dates for the other eclipses are rounded from actual records.
The ramp down to minimum and ramp up to maximum are quite smooth
and linear on a plot of magnitude versus date. The start and end of
the ramps are sharp within a week or two. The two ramps are NOT
symmetrical! 'Middle of eclipse' and 'middle of totality' are NOT on
the same date! Make sure you know which is intended in your reading.
The minimum is flat with one curious anomaly noticed in recent
rounds. There was noticed in the 1955-1957 and for sure in the 1982-
1984 round that at mid totality there was a spike of brightening of
about 0.2 magnitude. This lasted about a month before collapsing to
the general flat base of minimum.
This peculiar spike led to a revision in the general model of the
star from two spherical globes to a flattened globe or a disc orbiting
the main star. This I elaborate below.
There is no secondary minimum between the eclipses, like in a
typical eclipsing binary star. This is likely because the secondary
emits so little light that there is none to block off when the bright
stars is in front.
Between eclipses the maximum is pretty stable at +3.03 magnitude.
However, with newer, more sensitive, photometry we see fluctuations
above noise of a few hundredths magnitude. Regular routine photometry
of epsilon in advance of the upcoming eclipse began in 2006.
Secondary component
-----------------
While the specs of the primary component seemed tame, those for
the secondary star were, uh, weird. In the 20th century it was treated
like a humongous dark sphere that simply took the two years to get all
the way across the primary star and had a 27-year orbit around it.
With newer instruments and techniques, this model proved
infeasible. The current working hypothesis is that the secondary is a
thick disc. There is a central star, obscured by the disc's material,
and a cleared 'eye' around that central star. The disc presumably
rotates, whence its creation and stability. The mass of the disc
material is assumed to be small compared to the central body, here
called a 'star'.
By good fortune the disc orbits edgeon to Earth to create what
looks like a central transit. It's not a complete occultation because
the disc is thinner than the primary's diameter. The polar sections
peek over and under the disc to shine toward Earth. This model could
explain why the spectrum is about the same within and without eclipse.
This does make the computation of models a lot easier but not
necessarily more correct. A schematic cross-section of epsilon Aurigae
is somewhat like this
dust disc _ central 'star'
| / \ | /
@@@@@@@@@ * @@@@@@@@@ - O - --F0-I supergiant
| \_central cavity / | \
| |
|<---------- 27.6 AU --------->|
The central cavity, cleared out of the disc by centrifugal force,
could account for the spike in brightness at mid totality. The
material in front of the primary is a bit less when the cavity passes
over it, letting a bit more light reach us.
The disc seems to be made of particulate matter that does not
produce ordinary spectral liens. The disc may be crooked, causing the
unequal durations of ingress and egress partial phase.
A few properties of the disc are:
+--------------+----------------------+
| parameter | value |
+--------------+----------------------+
| 'star' mass | 15 Sun |
| disc mass | 5 Sun |
| disc radius | ~2,000 Sun, ~10 AU |
| orbit radius | 27.6 AU |
| orbit excent | 0.2 |
| orbit period | 27.12 year |
+--------------+----------------------+
The mass of the two components is about equal, so the center of
gravity is quite halfway between them. This allowed for a simpler
analysis of the radial velocity oscillations taken from the primary
star.
One common version of this model has a tilted disc that lets us
see thru the central hole. When the disc is transiting the primary
star, the star shines thru the hole to cause the midtotality
brightening.
The hole in this case must still be filled with obscuring
material, tho thinner than elsewhere, because the increase in
brightness is only 2/10 magnitude. Out of eclipse we have not seen
anything of the central concretion, the 'star', in the middle of the
hole. It could be that the central body is not a regular star.
Do understand well that this disc model is just that, a model, and
is NOT the proved solution for the star.
Observing
-------
Epsilon Aurigae is a bare-eye star near Capella. It is readily
spotted unless the sky is severely illuminated or veiled by haze or
thin cloud. In these cases, binoculars are needed.
Familiarize your self with Auriga and its surrounds, thru a
planetarium computer program or other means, so you can canfidently
identify Capella where ever you may be outdoors at the instant. It is
necessary to know your compass directions or box in Capella among
other rcognizable stars.
Epsilon Aurigae in August 2009 is coming into the night sky in the
northeast for the New York latitude. It there after is an night to
evening feature thru April or so of 2010. By good fortune in 2009 the
star is in good view during its first partial phase of the eclipse.
The brightness changes are slow, with time scale of a week or so,
and the minimum phase lasts quite two full years. frustration and
impatience can creep over you. The star isn't doing anything!
You need inspect the star only once a week as the weather and
earthly circumstances allow. Do not accumulate masses of observations
like several during one night as if epsilon was a flare star. Once a
week is plenty for any one person.
It's best not to try for a clockwork schedule of observation. It
WILL quickly get out of order from weather and other busyness.
However, you may find that you in your daily routine can stop for a
few minutes at a suitable viewing spot, Inspect epsilon Aurigae. and
continue on your way. If your daily routine permits, wait until Auriga
is in high sky. During winter, Auriga is about overhead at nightfall.
Epsilon's range of magnitude is annoyingly small, from 3.00 at
maximum outside eclipse to 3.83 at minimum. You must take care in
boxing the star between the brightness of surrounding stars to get a
reading within 0.1 magnitude. This sounds tough for the newcomer but
it can be achieved with a bit of practice.
At first you will find it slipper to assign a value to epsilon.
You can say only that it's somewhere between two particular comparison
stars. Your first impression usually is your best. Staring at the
stars for several minutes will frustrat you endlessly.
In spite of the far superior devices now ramping up their study of
the star, the home astronomer with eye and binocular is still a vital
element for collecting data. Each individual observation by a home
astronomer counts!
Comparison stars are a bit troublesome due to the overall
brightness of epsilon. For a bright variable star there just are fewer
other bright stars around it. Here are several in or near Auriga. Spot
these stars on a staratlas.
+----------------+-------+
| star | magn |
+----------------+-------+
| epsilon Persei | +2.89 |
| delta Persei | +3.01 |
| eta Aurigae | +3.17 |
| epsilon Tauri | +3.53 |
| delta Aurigae | +3.72 |
| zeta Aurigae | +3.75 |
| nu Aurigae | +3.97 |
| xi Persei | +4.04 |
+----------------+-------+
The technique sounds simple. Find two stars that box epsilon, one
a bit brighter and one a bit dimmer. The estimate 'how much' in
between in brightness is epsilon. Is it more like the brighter star?
About halfway between in brightness? A bit dimmer than halfway? Then
interpolate for the magnitude value between the magnitudes of the
comparison stars.
You should try other pairs of comparison stars, not just one
certain two of them. You assessment should agree closely. If
uncertain, supply a report for each pair, to be safe.
When Auriga is low in the sky, try to pick stars near the same
altitude as epsilon to minimize the effects of attenuation near the
horizon. This ma not be possible at the instant, so qualify the
observation with you comments.
The information in your report is:
+-------------------------------------------------------+
| your name and contact, such as email or telephone |
| observing location by town & state or district & city |
| optic such as bare eye or binocular specification |
| sky transparency, haze/cloud, Moon, smoke |
| date and nearest hour with timezone |
| assessed brightness of epsilon within 0.1 magnitude |
| comparison stars employed |
| comments about the observation |
+-------------------------------------------------------+
Your report may be pusted directly into the NYSkies yahoogroup
with subject 'eps Aur obsn' or sent to me. I'll capture it and lay the
data out in AAVSO form. As the reports accumulate I'll post summaries
of them in the yahoogroup or in a new page in our website.
It doesn't matter if you already sent your report to an other club
or to AAVSO. Please TELL ME so I don't send it to AAVSO gain as a
duplicate! Note in the comments: 'sent to AAVSO via XYZ Astro Club'.
Starbrowsing
----------
While you're in Auriga, please visit its other sights, the three
bright open clusters of M36, M37, and M38. About 1/2 from Elnath to
Capella is a pretty asterism, no special name, that fills the
binocular field. Auriga sits across the Milky Way with its peppering
of little stars along it.
The adjacent constellations of Gemini and Perseus have much to
see, too. Mars is in western Gemini heading east for his opposition.
It's a far opposition with only a tiny pink disc in small scopes. He
passes near M35, the major open cluster in Gemini.
Perseus also is in the Milky Way to offer fields full of stars.
The area around Mirfak is a pretty group and to the northwest is the
Double Cluster. M34 open cluster is near Algol.
To the north of Auriga is Camelopardalis (sometimes called
Camelopardus) with the spectacular Pazmino's Cluster. Details for all
of these targets, and for others, are in your observing guides.
Other methods
-----------
A higher level is to photograph the field of epsilon with a
CCDgraph and take the pixel measurements from the image. Comparing it
to those for other stable stars yields the magnitude of epsilon. This
is a more demanding method. It requires a camera and scope suitable to
capture celestial images and the patience and care to process the
images.
A still higher level is to use a regulation photometer and collect
the electric current generated by epsilon and its neighbor stars. The
analysis of the amps or volts yields the magnitude of epsilon. This
technique calls for good general radio-electronic skills and arts.
This is a faculty rapidly fading from our society since the late 20th
century.
An other option is to ally with a campus facility that needs
assistance like a lab technician. Apply at your local observatory or
astronomy lab for possible opportunities. Please note that you may be
asked to work without pay or only a stipend. That's all the more so in
2008-2010 when the world financial situation is at an ebb. Such work
may seem silly and boring, keying numbers into a database, cleaning
apparatus and tools, checking weather reports, fetching office
supplies.
If you can, by your means and manner, do this work, DO IT. You'll
look back on those months with intense pride. When you see the
published results of your lab, you'll think, 'Yes, I remember that. I
worked there'.
Daytime
-----
Virtually all variable star observations are done in night or, at
worse, twilight. Epsilon Aurigae is one rare exception for being a
bright star. With CCD-based photometers of the sort within reach of
home astronomer skill, this star can be monitored during the daytime.
It likely will not be visible in telescopes by eye alone.
Hence, from late spring to late summer, when epsilon Aurigae is in
strong twilight or in daylight, observations can continue. They would
fill that annoying hole, the 'solar lacuna', that punctuates most
variable star records.
I can't think of any spacecraft that will routinely image the
field of epsilon Aurgiae in the course of its other work. If so, you
could collect the images and make at least crude assessments of the
star. This technique is under experiment for SOHO pictures when the
target star is captured in the field with the Sun, being then near
conjunction.
Other studies
-----------
Why bother with home observations when there are observatories
examining epsilon Aurigae? Observatories can only allot a few minutes
to the star packed among the other targets on its schedule. Some are
of greater priority and can even bump epsilon off the list for a given
night.
Space-based observatories are studying the star almost entirely
outside the visual waveband, which is, after all, why they are placed
in outer space. Very few pure visual measurements are planned from
space, not even with Hubble. Even if there was time to look at
epsilon, it's only once in a long while among other targets.
This year, with far more and better instruments in orbit and in
deep space, spacefarers have a giddy time with epsilon! It can be a
major feature to point out and explain the value of space exploration
for studying it.
Home astronomers provide the numbers and frequency of inspection
that fills in a continuous record of the star's behavior, at least
within this one waveband of visible light. Your own peculiar report is
blended with hundreds of others within the same night to smooth out
errors and construct an amazingly detailed history for the star. Altho
you should shoot for the 1/10 magnitude step of accuracy, the
statistics of the pooled data can be a few hundredths of a magnitude
step.
That's very important! In previous eclipses and on & off between
eclipses the main star appears to have minor oscillations of
brightness. Not for sure but naggingly suspiciously so. For a single
observer, like you, the star away from eclipse looks pretty static. By
aggregating many observers, each at a different equipment and
longitude, these tiny variations show up. Are they real or just noise?
Julian day number
---------------
In your reading about epsilon Aurigae you come across 'Julian day
number' or 'Julian date'. Because variables stars and many other
astronomy phaenomena span long times, calendar arithmetic can be very
clumsy and uncertain.
To have a continuous uniform flow of time we employ the Julian day
number, a system invented in the 1600s but at that time it didn't win
much support. The zero day is in 4713 BC, which by the thinking of the
17th century predated the creation of the world. Hence, all day
numbers would be positive numbers.
It wasn't until variable star astronomy got going in the late
1800s that it came into wide use. It is also used for orbit
calculations, specially for comets and asteroids. Osculating elements
for asteroids and comets are generally cited for even 400-day
intervals of Julian day number.
A couple points to bear in mind about Julian day numbers. First,
it has nothing to do with the Julian calendar or Julius Caesar. Just
take it as the name of the numbering scheme.
Julian day numbers are banked off of Universal Time, NOT your
local timezone. You MUST first shift your clock reading to UT OR use a
chart with the zone shift built in.
Julian day numbers start at 12:00 UT, NOT midnight. This is to
lessen the chance of missing a date rollover during a night of
observation. The whole night is within one Julian date.
Finally, Julian days are decimalized for parts less than a day.
There is no adjustment for leapsecond, like the one coming at the end
of 31 December 2008. Astronomers in the past assigned a decimal for
the leapsecond midway between that for the adjacent seconds.
Please note that while civil practice assimilates the leapsecond
into the local New Year's celebration, the leapsecond is formally
added at 23:59:59 UT. For EST, it comes at 18:59:59.
Here is a table of Julian day numbers for the zeroth day of each
month for the apparition of epsilon Aurigae. To get the Julian date
for a given calendar date at 19h EST, add together (2,450,000), the
Julian day number of the zero day, and the day within the month.
+-------------------------------------------+
| JULIAN DAY NUMBER FOR 0TH DAY AT 19H EST |
+-------------------------------------------+
| month | 2008 | 2009 | 2010 | 2011 |
+-------+--------+--------+--------+--------+
| Jan 0 | 4465.5 | 4831.5 | 5196.5 | 5561.5 |
| Feb 0 | 4496.5 | 4862.5 | 5227.5 | 5592.5 |
| Mar 0 | 4525.5 | 4890.5 | 5255.5 | 5620.5 |
| Apr 0 | 4556.5 | 4921.5 | 5286.5 | 5651.5 |
| May 0 | 4586.5 | 4951.5 | 5316.5 | 5681.5 |
| Jun 0 | 4617.5 | 4982.5 | 5347.5 | 5712.5 |
| Jul 0 | 4647.5 | 5012.5 | 5377.5 | 5712.5 |
| Aug 0 | 4678.5 | 5043.5 | 5408.5 | 5773.5 |
| Sep 0 | 4709.5 | 5074.5 | 5439.5 | 5804.5 |
| Oct 0 | 4739.5 | 5104.5 | 5469.5 | 5834.5 |
| Nov 0 | 4770.5 | 5135.5 | 5500.5 | 5865.5 |
| Dec 0 | 4800.5 | 5165.5 | 5530.5 | 5895.5 |
+-------+--------+--------+--------+--------+
Example: Julian day number for 15 February 2009 at h EST:
(2,450,000 base)
+ (4,862.5 from table)
+ (15 day within month)
= (2,454,877.5)
The decimal part of the Julian date is taken from the table below.
For observations of epsilon Aurigae, times are needed only within an
hour. The table covers only the dusk thru dawn hours in New York.
+-------------------------------+
| DECIMAL OF JDN FOR UT AND EST |
+---------+-----+---------------+
| decimal | UT | EST = UT-5h |
+---------+-----+---------------+
| 0.3750 | 21h | 16h same date |
| 0.4167 | 22h | 17h same date |
| 0.4583 | 23h | 18h same date |
| 0.5000 | 00h | 19h prev date |
| 0.5417 | 01h | 20h prev date |
| 0.5833 | 02h | 21h prev date |
| 0.6250 | 03h | 22h prev date |
| 0.6667 | 04h | 23h prev date |
| 0.7083 | 05h | 00h same date |
| 0.7500 | 06h | 01h same date |
| 0.7917 | 07h | 02h same date |
| 0.8333 | 08h | 03h same date |
| 0.8750 | 09h | 04h same date |
| 0.9167 | 10h | 05h same date |
| 0.9583 | 11h | 06h same date |
| 0.0000 | 12h | 07h next JDN |
| 0.0417 | 13h | 08h next JDN |
+---------+-----+---------------+
If you observe at 22:40 EST on 15 February 2009, the complete
Julian date is:
(2,454,877.5 from above)
- (0.5 to kill the decimal for 19h)
+ (0.6250 for 22h)
= (2,454,877.6250)
The extra 40 minutes within the 22nd hour are ignored. If you prefer,
you may round your time to the hour, in which case the Julian day
number is 2,454,877.6667.
The two most common mistakes in reporting Julian date is
neglecting the timezone shift and forgetting the rollover at 12h UT.
Many computer astronomy programs have a calendar function that
produces the Julian day number for an input date and hour. Study the
instructions to know if the entered date is treated in UT or in the
timezone which the program is set for. Adjust the input accordingly.
Support
-----
For the simple task of assessing epsilon's brightness with the
eyeball method, you need the comparison stars. AAVSO has charts of
Auriga and surrounds with many stars labeled with their magnitudes.
It is precisa mente because of epsilon's overall brilliance that
there are few good comparison stars nearby! As a star is dimmer, there
are more, a lot more, other dim stars around it. From this greater
number of stars some good comparison stars can be selected.
Not so at the bright end. There will have to some relaxation from
some of the rules of variable star observing. It may, for instance, be
hard to find stars of about the same altitude as epsilon, to avoid
different air-mass absorption among them. Auriga as a whole sits next
to a vast desert of dim stars in Camelopardalis and Lynx, closing off
the north for comparison stars. AAVSO and other observing centers will
have to make some hard choices for the charts.
Note, too, that a chart intended for CCDgraphy or photometry can
NOT be used for eyeball measurements. The magnitudes on these charts
are fitted to the spectrosensitivity of these instruments. This is NOT
the same as that of the eye.
Citizen science
-------------
I'm hearing lavish plans to incorporate the layfolk in collecting
observations of epsilon Aurigae. The dream seems to be a mass flow of
people outdoors each clear evening armed with starcharts and
notebooks. They inspect epsilon, scribble their measurement, and later
send the record to a central clearinghouse. This could be your
astronomy club.
As much as it sounds wonderful to involve more people into our
profession, there is the utterly uncontrollable hazard of collecting
really dirty data. In the annual 'GLOBE at Night' assessment of sky
illumination, the information turned in by layfolk ends up being so
useless and irreparable that it has to be disregarded. In that
project, much of the trouble was in the online data report method and
lack of validation and correction.
However, as was the experience in New York with Top of the Lawn,
lay people can make useful contribution while under supervision of
practiced astronomers. This was done at TOTL's sessions in Central
Park, Manhattan. Experienced skywatchers noted sky transparency of 4-
1/2 magnitude while the general public could be sure of only 3-1/2. If
these folk were on their own, without correspondence with TOTL, the
data in GaN would be so pessimally skewed that no advocate against
luminous graffiti could rely on it.
A similar situation can happen with unstructured employment of the
general public for epsilon Aurigae. Yes, school kids will take the
charts home, mark them according to what they say they saw. Teacher
will collect the charts and send them to the clearinghouse. There they
meet their fate in the recycle basket.
If your club engages the public for the epsilon Aurigae program,
keep it under your control. At starviewing sessions stand with the
person, ensure that he is looking at the right star. Is he holding the
chart in the right orientation? Is he reading the magnitude numbers
correctly? Is he picking suitable comparison stars? Is he properly
doing the proportion estimate? Did he write down the proper numbers?
For an attendance of a dozen at your session, you CAN obtain good
public participation AND secure good astronomy data.
The validation for the public is seeing their data published, at
the very least, in your club newsletter or website. They can show it
off to friends and be better inclined to support the profession in
future years.
Litterature
---------
Litterature from previous eclipses are transferred to various
websites and turn up during web searches. Some is aimed to home
astronomers with small means, like only binoculars. Some assumes a
fixed telescope with photometers and imaging apparatus. Other articles
are technical reports from various journals. Sometimes a single piece
has both home and campus topics together, which may dissuade the less
experience home astronomer.
Please do look over articles relating to previous eclipses. The
27-yr period of epsilon Aurigae allows major evolution of instruments,
methods, study between eclipses, making for intriguing 'snapshots' of
the state of astronomy at each. For instance, the 1955-1957 just
missed the space age with only ground observations collected. The
1982-1984 round enjoyed space-based participation but missed Hubble,
FUSE, Spitzer and other major observatories.
It is also intriguing to note that at each eclipse home
astronomers made use of instruments available only to the campus
astronomer at the previous eclipse.
Don't shy away from what looks too complicated to read at first.
Set it aside and ask about it at the next club meeting. With epsilon
Aurigae a central theme for IYA, other club members are going to find
the same piece and maybe could explain it to you.
References
--------
I give here a couple websites to start your investigation of
epsilon Aurigae. Besides native material there, the pages have links
to other source material. Mind well that the titles or descriptions
could lead you to items of little immediate use for you.
Specially with the next eclipse coming in summer 2009 and IYA in
January 2009, the amount of epsilon Aurigae litterature is sure to
blossom rapidly.
www.hposoft.com/Campaign90.html - Hopkins-Phoenix Observatory
www.aavso.org - American Association of Variable Star Observers
www.du.edu/~rstencel/epsaur.htm - Dr Robert Stencel
These websites have many pages and links within them. Explore
around a bit. Because the material at these and other places is so
thoro, I purposely did not try to copy it into this article. The
topics I discuss here are tailored specificly for the home astronomer
You should also search the web for ('epsilon Aurigae' + 2009) for
additional references, like from the astronomy journals. ('epsilon
Aurigae') by itself will get you hundreds of hits for constellations,
interesting stars, skymaps, starfields mixed in with the good meat you
really need and want.
Link to the past
--------------
Epsilon Aurgiae illustrates one of the incredible awesome aspects
of astronomy. You can bond to your ancestors in the profession by
looking at the same targets and phaenomena they did. Recent examples
include Halley's comet and gamma Virginis.
Few other endeavors for home pursuit allow such a lineage across
the centuries and millennia. By watching the eclipse of epsilon
Aurigae you are linking to your parent's experience a quarter century
ago. They may have left you their notes, pictures, scrapbook about the
star. Now it's your turn to add to human advancement with your own
record.
When you read about the puzzlement of astronomers at the 1874 round
and see how the star is still a deep mystery today, you can hope that
by your observations some of that mystery will be stripped away.
Spectrometry
----------
Home astronomers have over the ages eventually acquired the
instruments and skills that campus astronomers enjoyed a generation
earlier. While only a minuscule fraction of home astronomer elevate to
the level to properly get good use from the new gadgetry, enough do to
produce solid data for the profession.
One field of campus astronomy so far eluded home astronomers,
spectrometry. As you read the reports of epsilon Aurigae you notice
the heavy emphasis on the star's spectrum. As yet there is no
substantial foundation for home astronomers to do spectrometry, even
of the vintage of the 1980s.
There are several reasons for this, not the least being the utter
absence of affordable workable credible spectrometers for home
telescopes. The ones on the markets are essentially toys or suitable
only for indoor lab use.
An other major cause for the lack of an active home spectrometry
practice is the highly technical and mathematical nature of the
subject. You need a good grounding in radiation, quantum theory,
thermodynamics, gas dynamics to properly interpret the spectrum.
Beyond collecting pretty colored strips of spectrograms, the home
astronomer at present just can not produce useful data for the
profession.
There are a few exceptions, but these are astronomers who work at
or personally have heavy-duty observatories. The Hopkins-Phoenix
Observatory in the references above is one example. It is hardly the
rig a typical home astronomer will have to hand.
In fact, spectrometers ceased taking 'pictures' of spectra decades
ago. They collect point by point measurements of the incoming flux at
each wavelength within their resolving power. The output is a computer
file of wavelength versus value. From this you must, with appropriate
software, synthesize a 'spectrogram' or other analytical output.
Conclusion
--------
The coming eclipse of epsilon Aurigae will be one of the more
awesome spectacles of your astronomy career! If you be a twenty-
something chap, it's the first chance of a lifetime. The sheer length
of the eclipse, with its colossal and gigantic dark 'thing' lumbering
in front ot the bright star, will freak you out.
The casual home astronomer can make valuable measures of the star
with just eye and chart or simple CCDgraphs. AAVSO welcomes your
contributions. The exciting period is the ramp down and ramp up in the
partial phases, but do keep an eye on the star near mid totality.
There could be a subtile rise from causes really not well known.