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EI2GYB > ASTRO 12.10.24 00:39l 120 Lines 9796 Bytes #999 (0) @ WW
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Subj: This Week's Sky at a Glance, October 11 - 20
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Sent: 241011/2231Z 34310@EI2GYB.DGL.IRL.EURO LinBPQ6.0.24
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This Week's Sky at a Glance, October 11 - 20
Comet Tsuchinshan-ATLAS (pronounced "Choo-chin-SHAHN") is entering its week of glory for those of us in the Northern Hemisphere. It has swung around the Sun and begins emerging into evening view around Friday the 11th, very low due west in twilight. Binoculars will help you pick it up through the fading afterglow of day.
Each day the comet gets higher in the west and sets later and later, displaying itself in plain view (we hope!) in late twilight and then, as the days pass, also after twilight ends. Moonlight will make for a less than ideally dark night sky, but hey. The comet's long tail - 10ø or more - will point more or less upward depending on your latitude.
FRIDAY, OCTOBER 11
Ý Bright comet low in evening twilight! Comet Tsuchinshan-ATLAS (C/2003 A3) swung through perihelion on September 27th, passing 0.39 a.u. from the Sun. Now, as its solar roasting continues, it emerges low into the Northern Hemisphere's evening-twilight sky with its head likely to be shining at around magnitude -1.5, as bright as Sirius.
Start trying for the comet this evening, but it might not yet be easy. Find a place with a clear view practically down to the horizon due west. By 30 minutes after sunset, start examining the sky there just 6ø or so above horizontal. That's hardly more than three finger-widths at arm's length.
Alternatively: The comet's head will be about 28ø - meaning almost three fist-widths - to the right of bright Venus, and probably somewhat lower (depending on your latitude; the farther south you are, the lower with respect to Venus).
It will likely appear tiny, with its bright inner tail curving sharply to the right tonight. The comet sets while twilight is still fairly bright.
Tomorrow it'll be about 4ø higher and a little easier. The view will continue to improve as it gets higher every evening after that, and the tail will swing around to point upward. However, even as Comet T-A becomes easier to locate, it will fade as it recedes from the Sun.
SATURDAY, OCTOBER 12
Ý Now Comet Tsuchinshan-ATLAS stands higher, about 10ø above the west horizon 30 minutes after sunset (as seen from mid-northern latitudes). Its brightness may be down to magnitude -0.8. Again, bring binoculars or a telescope if you have them.
Sixty minutes after sunset, as a few stars are beginning to come out, the comet's head is still 5ø above the horizon's west point. The head sets around twilight's end, but as darkness becomes complete the long dust tail may be detectable extending very far upward - depending on your light pollution. Once night is fully dark, how much of the tail can you see emerging from the west horizon?
SUNDAY, OCTOBER 13
Ý And now, even 60 minutes after sunset, the comet 's head is still a fine 9ø high or so in the darkening sky. It's about midway between Venus to its left or lower left, and Arcturus to its right or upper right. The head may match Arcturus, magnitude zero, for brightness.
By now T-A is predicted to grow a thin, faint antitail pointing opposite the main tail. An antitail can appear when Earth passes through the plane of a comet's orbit and we view the thin, broad sheet of its dust debris edge-on.
At twilight's end now, the comet is still a couple degrees over the horizon. However, when any celestial body is that low, atmospheric extinction dims it considerably.
The comet will climb higher and stay up longer through the rest of the coming week, while dimming and receding into the distance. Expect it to be about magnitude 2.5 on the evening of the 16th and mag 4 by the 21st.
The post-twilight sky grows poorer as the light of the waxing gibbous Moon grows brighter. The Moon is full on the evenings of the 16th and 17th. But on about the 20th, a window of darkness will begin to open between twilight's end and moonrise.
MONDAY, OCTOBER 14
Ý How early in twilight can you spot Saturn, magnitude +0.7, in the far background of the Moon? They'll appear about 5ø apart during evening in North America. See the scene above.
TUESDAY, OCTOBER 15
Ý This evening for the Americas, the head of Comet T-A will be passing only about 1.2ø south of the 6th-magnitude globular cluster M5 - "a not-to-miss opportunity for photographers," writes Bob King. The comet's head by now should be about 2nd magnitude.
WEDNESDAY, OCTOBER 16
Ý Full Moon this evening and tomorrow evening. The Moon is exactly full at 7:26 a.m. EDT on the 17th: about halfway between these two evenings for the Americas.
Ý And while comet-spotting, take note of Arcturus low in the west-northwest about 2« fists at arm's length to the comet's right or lower right. Sometime around when twilight ends, you'll find that Arcturus is the same height above the horizon as equally bright Capella in the northeast. When that happens, turn to the south-southeast, and there will be 1st-magnitude Fomalhaut at about the same height too - exactly so if you're at latitude 43ø north (Boston, Buffalo, Milwaukee, Boise, Eugene). Seen from south of that latitude Fomalhaut will appear higher; from north of there it will be lower.
THURSDAY, OCTOBER 17
Ý The Great Square of Pegasus, emblem of autumn, is now high in the east-southeast after dark - still, for now, balanced on one corner (for the world's mid-northern latitudes).
FRIDAY, OCTOBER 18
Ý This is the time of year when the Big Dipper lies down horizontal low in the north-northwest when evening grows late, around 10 p.m. How low? The farther south you are, the lower. Seen from 40ø north (New York, Denver) even its bottom stars twinkle nearly ten degrees high at their lowest. But at Miami (26ø N) the entire Dipper will skim along out of sight just below the north horizon.
SATURDAY, OCTOBER 19
Ý The Moon, now waning gibbous, follows a few degrees behind the delicate Pleiades as they cross the sky tonight. Use your fingertip to block the Moon's glare.
Aldebaran and Jupiter follow along farther behind the Moon.
SUNDAY, OCTOBER 20
Ý Now that it's mid-October, Deneb has replaced Vega as the zenith star after nightfall (for skywatchers at mid-northern latitudes). Accordingly, Capricornus has replaced Sagittarius as the zodiacal constellation posing low in the south.
Ý The Orionid meteor shower should be active in the hours before dawn Sunday morning, but don't expect to see many at all through the waning gibbous moonlight. The radiant, in Orion's upraised club, will be high in the southeast to south by then.
Ý Even if the meteors disappoint when you're out before dawn Sunday, you can certainly catch Pollux, Mars, and Procyon forming a straight line, in that order from upper left to lower right. The three shine with fairly similar brightnesses: magnitudes +1.1, +0.3, and +0.4, respectively. Pollux is three times closer to Mars than Procyon is.
Mars is moving eastward against the stars fast. Tomorrow morning the line won't be so straight.
This Week's Planet Roundup
Mercury remains out of sight deep in the sunset.
Venus, magnitude -3.9, gleams low in the southwest as evening twilight fades. It sets around twilight's end.
Mars (magnitude +0.3, in Gemini) rises around 11 or midnight. It shows best, very high in the southeast, in the hour before dawn. It's now 30ø down east of bright Jupiter. Mars in a telescope is still a small 8 arcseconds wide, on its way to a mediocre opposition (14.5 arcseconds) next January.
Jupiter (magnitude -2.6, still near the horntip stars of Taurus) rises in the east-northeast around 9 p.m. Like Mars, it's highest in the hours before dawn. Jupiter is now a nice 44 arcseconds wide in a telescope, hardly smaller than the 48-arcsecond width it will attain for the weeks around its opposition in December.
Uranus (magnitude 5.6, near the Taurus-Aries border) is well up by late evening about 5ø from the Pleiades. You'll need a good finder chart to identify it among its surrounding faint stars.
Neptune (tougher at magnitude 7.8, near the Circlet of Pisces) is 14ø east of Saturn. Again you'll need a proper finder chart.
All descriptions that relate to your horizon - including the words up, down, right, and left - are written for the world's mid-northern latitudes. Descriptions and graphics that also depend on longitude (mainly Moon positions) are for North America.
Eastern Daylight Time (EDT) is Universal Time minus 4 hours. UT is also known as UTC, GMT, or Z time.
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