Mercury

Late September is the best time to see the innermost planet in the morning sky this year, at least from northern latitudes. The steep angle of the ecliptic swings Mercury up as high as it gets in the eastern sky, with Mercury's maximum elongation away from the Sun coming on September 19. Around that date, look for a zero-magnitude "star" shining in the twilight about 5 to 10 degrees (one to two binocular fields) above the horizon. Aim a telescope at it (inset), and you'll see Mercury appearing as a world at half phase.

courtesy starry night pro plus,m/simulation curriculum corp

What's more, because Hartley's inclined orbit keeps it above the plane of the Earth's orbit in September and for most of October, we see the comet high in the northern constellations Cassiopeia and Perseus.

From Canada, the comet is actually circumpolar and already high in the eastern sky as it gets dark each evening. By 3 a.m., local time, Hartley shines almost straight overhead as it travels down the scenic Milky Way in Perseus. We couldn't ask for better viewing geometry. Now we just need a bright comet sporting a prominent dust tail.

For updated information on Comet Hartley's predicted brightness, see Japanese comet observer SeiichiYoshida's web-page at www.aerith.net/comet/catalog/ 0103P/2010.html. For actual reports of the comet's current brightness, see the International Comet Quarterly page at www.cfa.harvard.edu/icq/CometMags. html.

equinox harvest moon

Every year, we are treated to the Moon of verse and song, the Harvest Moon, heralding the change of seasons and the golden harvest. This year, the Harvest Moon coincides with the equinox. For the northern hemisphere, autumn begins on September 22 at 11:09 p.m., EDT.Just six hours later, the Moon is officially full and directly opposite the Sun. So if you want to witness the true Harvest Moon, wait until dawn (about 7 a.m., local time) on Thursday, September 23, when the Moon appears as full as it will get for the night and is setting almost due west. Photographers looking for a great shot of the Moon over some landmark in the west should plan to get up early on September 23.

Most of us, however, will be happy to watch the Moon at a more convenient time, 12 hours earlier, on the evening of Wednesday, September 22. It will then rise almost due east, with the Sun still up in the west.This is a great opportunity to catch an image of the Moon rising behind a skyline, with buildings reflecting the golden light of the setting Sun.

If you miss it on the Wednesday night, you'll have another chance the following evening. Because of the shallow angle of the ecliptic with respect to the horizon at this time of year, the Moon isn't much farther below the horizon the next night.

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