A Daring Pairing of Moon and Venus
Early risers in North America are in for a treat on the morning of Wednesday, April 22nd, when the waning crescent Moon passes in front of the brilliant crescent Venus.
Tracks for Venus occultation
The apparent path of Venus behind the Moon on April 22, 2009, depends on where you are. The local horizon is approximately downward; celestial north is to the upper left.
Sky & Telescope
For much of the continent the occultation happens after sunrise in broad daylight. If the air is clear you can find the thin, dim Moon about 33° to the upper right of the early-morning Sun, with Venus near its edge. Sadly, this event will be a near miss along the Eastern Seaboard.
I'm tempted to hop a westbound plane, because in the Far West the occultation happens during morning twilight — with the Moon and Venus shining beautifully before sunrise but very low above the eastern horizon. When the Moon covers Venus they'll be only 6° up as seen from Los Angeles, 4° from San Francisco, and 6° from Seattle. When Venus reappears at these locations the Moon will be about 10° higher, with the Sun already risen or nearly so.
Because the Moon is waning, its bright edge will be the one that covers Venus and its dark limb will uncover it, as shown in the diagram above. The thin crescent Moon will be only 9% sunlit, while Venus (40 times smaller) will be a 17% crescent.
At locations where the sky is still fairly dark, all you'll need are your eyes to watch this celestial spectacle. You'll probably need binoculars or a telescope where the sky is bright. Because of Venus's significant angular size, its disappearance and reappearance will each be gradual, taking 30 seconds or more.
You can gauge the approximate times of Venus's disappearance and reappearance at your site using the maps below. Interpolate between the red time lines to get the Universal Time of the event. Along the graze line, you'll see the Moon's southern limb skim Venus but never quite cover it. The maps also indicate whether your location will be in twilight or daylight at this time. (You can get customized predictions for several hundred cities and towns here.)
Maps of Venus's occultation
Use these maps to estimate the Universal Times of Venus's disappearance and reappearance during its occultation by the Moon on April 22, 2009.
Sky & Telescope diagram
And if you're willing to get up a few hours earlier, you might be treated to a nice smattering of "shooting stars" from the annual Lyrid meteor shower. In most years you'll see fewer than 20 Lyrids per hour before the first light of dawn even under perfectly dark skies. But some years have brought outbursts of up to 90 per hour. Will 2009 be one of those?
A small 8th-magnitude comet is now making its way slowly across Cassiopeia toward Perseus. The surprise visitor, called Comet Yi-SWAN, should be within reach of small telescopes for most of April and May 2009. However, bright moonlight will make it a challenge to spot until the second half of April.
So far, observers describe the comet as a diffuse glow just a few arcminutes across. No one has yet reported a tail, but that could change in the coming days as the Moon's glare wanes.
Because of its high declination, Comet Yi-SWAN is theoretically visible all night from mid-northern latitudes. But early rising skywatchers have the best chance, because Cassiopeia is highest during the predawn hours.
On Saturday evening, April 11th, the comet passes just 0.4° south of Schedar (α Cassiopeiae), so they'll be in the same low-power telescopic field. On Thursday night, April 23-24, the comet will glide only 1.2° south of the Double Cluster in Perseus.
The Discovery
Korean amateur Dae-am Yi was the first person to notice the comet. On March 26th, he obtained two images of it with a Canon 5D camera and 90-mm lens. But word was slow to reach the Central Bureau for Astronomical Telegrams (CBAT) at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, the world's clearinghouse for comet discoveries.
Independently, Rob Matson reported to CBAT on April 4th that he'd found a possible comet in SWAN images posted on the SOHO website.
Recognizing that the objects were one and the same, CBAT astronomers announced the find on IAU Circulars 9034 and 9035 and gave it the name Comet Yi-SWAN (C/2009 F6). Preliminary calculations by Brian G. Marsden indicate the comet is traveling in a highly inclined parabolic orbit, tipped 85.7° to the plane of the ecliptic. At perihelion on May 8th, the comet will be 1.27 astronomical units (190 million km) from the Sun and half again farther from Earth.
The ephemeris below, from Marsden's orbital elements, gives the comet's right ascension and declination (equinox 2000.0) at 0 hours Universal Time on selected dates, along with its elongation angle from the Sun, predicted visual magnitude, and the constellation in which it lies.