October stargazing events | Natural History

[Posted by Chuck Almdale]

Joe Rao writes a monthly column in Natural History Magazine, and I’ve mined it for information for many years. He characterizes October as “the best month for stargazing,” so I’m passing on some of his comments to you. If you’re lucky you have a dark sky in your area, and can see more than the couple-of-dozen stars available to us here in in light-saturated Los Angeles. A few additional comments from me are in [brackets].

Quiz time: If you’re standing at the South Pole (as in the picture above), how many “days” (start of sunrise start to next start of sunrise) will you have during the course of a calendar year? Answer at end.

Mercury: It’s always difficult to see this fast-moving, close-to-the-sun messenger of the gods. [Many earthlings have never seen it.] It’s highly elliptical orbit is only 88 days long, and the planet is 50% farther from the sun at aphelion [43.5 million miles] than at perhelion [28.5 million miles]. [Orbital mechanics dictates that it spends a lot less time close to the sun than farther away. From our viewpoint, it’s never more than 28° from the sun (56 times the width of full moon).]

9 Oct: Mercury passes inferior conjunction between the sun and earth, and passes into the morning sky. By the 17th it will be of 1st-magnitude brightness and will rise just south of east more than an hour before the sun.

25 October: Mercury reaches greatest western elongation and is magnitude -0.6 this morning. Although only 18° from the sun [36 times width of full moon], for several days surrounding this date the planet will rise before the beginning of morning twilight, making this the year’s most favorable morning apparition for observers in mid-northern latitudes. Brightening to -0.8, Mercury will pass 4° to the left of sparkly Spica on 2 November.

[Note on star magnitudes: The scale is reverse logarithmic: the brighter an object is, the lower its magnitude number. For example, a star of magnitude 2.0 is 2.512 times brighter than a star of magnitude 3.0, 6.31 times brighter than a star of magnitude 4.0, and 100 times brighter than one of magnitude 7.0. Wikipedia]

Saturn is the bright yellow “star” in the south during early evening. It lies on the western side of the dim, boat-shaped Capricornus, which is composed of 3rd and 4th-magnitude stars that Saturn, along with brilliant Jupiter to the east, overshadow. Saturn ceases its westward or “retrograde” motion (the direction all outer planets appear to move for a few months around opposition) and resumes its eastward travel against the stars. [Check your handy epicycle charts for details on retrograde motion.]

The moon is new on 6 Oct. at 4:05 a.m. PDT, at first quarter on 12 Oct 8:25 p.m. PDT, full “Hunter’s” moon on 20 Oct 7:56 a.m. PDT, and last quarter on 28 Oct 1:05 p.m. PDT.

Mars is in solar conjunction and invisible all month.

Venus is resplendent at magnitude -4.6, arrives at its greatest eastern elongation, or greatest angular distance east of the Sun (47°). But it is so far south on the celestial sphere that it remains fairly low, just 12° above the southwest horizon 45 minutes after sunset.

Quiz answer: One “day” sunrise to sunrise, per calendar year. The sun begins rising at the South Pole around 21 Sept (Autumnal equinox), begins setting around 21 March (Vernal equinox), then rises again at the next Autumnal equinox. It takes a few weeks to fully rise or set.