Week-by-Week Viewer's Guide
for April 2002
Skip to April
24 through May
April 5 through 23
This will likely be the
best time to look for Ikeya-Zhang as it slowly fades from third to fifth magnitude.
In the evening hours, the comet will be found roughly 10 degrees
above the north-northwest horizon about an hour after sunset.
Morning will offer the best
viewing opportunities, however. The Moon rapidly becomes less and less of a
factor after it passes Last Quarter on the 4th, again sliming down
to narrow crescent. By the 8th, the skinny sliver of the Moon is
rising just more than an hour before sunrise, low in the east-southeast, and
so it is no longer a hindrance to comet watching.
Meanwhile, the comet itself
rapidly heads up into a dark predawn sky, fully escaping both moonlight and
twilight after the first week of April. The moonless (or nearly moonless) mornings
from April 9-23 will likely see Ikeya-Zhang at its very best.
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What time should you set
your alarm clock for? To be safe, plan on getting to your prospective viewing
site at least two hours before sunrise, or perhaps even earlier if you live
north of 45 degrees latitude.
The comet will continue
to scoot northward, so far north in fact, that eventually it will become "circumpolar"
for many localities. This means that it will approach close enough to the north
celestial pole to allow it to be continuously visible all night long. For latitude
45 degrees, Ikeya-Zhang will become circumpolar April 7; for latitude 40 degrees,
April 10; for latitude 35 degrees, April 14, and for latitude 30 degrees, April
18.
From April 9-16, let the
bright zigzag pattern of stars that form the "W" shape of Cassiopeia
guide you to Ikeya-Zhang. The comet, probably shining at around fourth magnitude,
will appear to be sailing below and to the right of this well-known set of stars.
On the morning of April
10, draw an imaginary line from the star Gamma Cass to the star Schedar (the
third and fourth stars in the "W" going from left to right). Go just
over 1½ times the distance between these two stars, and you will be brought
to the comet.
Then, on the morning of
April 16, draw an imaginary line again from Gamma Cass, up to the star Caph
(the fifth star in the "W" going from left to right). Go just a little
more than the distance between these two stars and you will again come to the
comet.
The Moon is at First Quarter
phase on April 20th. After that, it will set later at night with
the "window of darkness" between its setting and the beginning of
morning twilight closing rapidly. By the morning of the 23rd, the
Moon is setting about the same time that twilight begins. Thereafter, moonlight
will again seriously compromise views of the comet.
April 24
through May
The Moon is Full on April
26 and then wanes to Last Quarter on May 4. Moonlight will wash out the comet
to some degree in the morning sky through this entire interval, but this is
no time to give up on Ikeya-Zhang.
Now flying tail-first, the
comet is racing toward Earth; its head will be passing closest on the evening
of April 29, when it will lie 37.6 million miles away, more than 2½ times closer
than it was in early March.
On this night, the comet,
now probably fifth magnitude, will be 15 to 20 degrees up
in the north-northeast sky an hour after sundown. The Moon, two days past full,
won’t come up for at least another couple of hours, at which time Ikeya-Zhang
has climbed another 10 degrees.
You might have your very
best view of the tail before the Moon begins to rise.
Even if the tail is lost
later at night in moonlight, the comet’s head should still be well worth examining
with a telescope. Doing so becomes increasingly easy as Comet Ikeya-Zhang continues
to gains altitude in the north-northeast morning sky, while crossing from the
constellation Cepheus into Draco on the 27th.
As it approaches us, the
comet's apparent night-to-night motion against the star background speeds up.
After April 29, the comet
will be traveling away from both the Sun and Earth so it will fade rapidly.
The waning Moon will become
less of a hindrance each night and you can look for Ikeya-Zhang well up in the
northeast sky as soon as darkness falls. Using a star chart, you might try and
follow Ikeya-Zhang through the month of May with binoculars or a telescope as
it dims from fifth magnitude at the beginning of the month, to eighth magnitude
by month’s end.
On the night of May 5-6,
it will be moving through the lozenge-shaped head of Draco and pass just over
a degree from the bright second-magnitude star Eltanin. It moves into Hercules
on May 8, Corona Borealis on May 18, and finally Serpens on May 25, where it
will remain into early summer, fading to eleventh magnitude, sailing back to
beyond the outer reaches of the solar system.
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Definitions
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Degrees
measure apparent sizes of objects or distances in the sky, as seen from
our vantage point. The Moon is one-half degree in width. The width of
your fist held at arm's length is about 10 degrees.
1 AU, or astronomical
unit, is the distance from the Sun to Earth, or about 93 million miles.
Magnitude is
the standard by which astronomers measure the apparent brightness of objects
that appear in the sky. The lower the number, the brighter the object.
The brightest stars in the sky are categorized as zero or first magnitude.
Negative magnitudes are reserved for the most brilliant objects: the brightest
star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest
stars visible under dark skies are around +6.
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