Moondark for November: Catching Mercury  
In the past few weeks, early risers have had a great opportunity to catch the planet Mercury.  Beginning about an hour before dawn, the closest planet to the Sun is visible as the morning star in the eastern twilight, the only bright “star” in that area of the sky. Well above it, though not nearly as bright are Regulus (representing the heart of Leo the Lion) and Saturn. As dawn approaches, Mercury disappears quickly into the rapidly brightening sky.

Mercury is elusive not because it is dim, but because you have to look at just the right time and in just the right part of the sky. Exactly where and when to look depend on orbital geometry, your geography and the angle the ecliptic makes with the horizon. On 22 October, Mercury was at western greatest elongation, its maximum apparent distance west of the Sun as seen from Earth. The ecliptic trends southward from Gemini, crossing the equator in Virgo, and that dip angles the ecliptic roughly perpendicular to our horizon around sunrise.

Under these conditions, Mercury is as far from the Sun as possible, at an angle well above our horizon, optimal conditions for seeing Mercury. In fact, this apparition is one of the most favorable for the whole year. At mid northern latitudes, Mercury is typically at its best as a morning star in the fall, while the spring favors Mercury as an evening star. This situation is reversed for observers in the southern hemisphere. Somewhat surprisingly, southern latitudes are overall better for viewing Mercury because of the planet's elliptical and inclined orbit (more so than all other planets except Pluto's) as well as Earth's tilted axis.

Even if you have a late commute from work, you won’t have to wait long to catch sight of Mercury. In late November, Mercury passes through superior conjunction, almost directly in line with the Earth and Sun on the far side. Recall that transits of Mercury occur only in November as well as six months later in May. Mercury then climbs to become an evening star as 2009 begins, swinging to its greatest eastern elongation in mid January. This time look before the planet sets about an hour after sunset, for which you’ll need a clear view of a low southwestern horizon on the commute home.  Mercury will again be an evening star, somewhat more favorably placed on its next trip around the Sun in late April and May.

If clouds block the view or if you can’t face the nighttime chill, you can always grab a laptop and browse Mercury from the comfort of a warm chair. NASA’s MESSENGER [MErcury Surface, Space ENvironment, and GEochemistry and Ranging) probe has returned stunning images of a cratered planet. Launched in 2004, MESSENGER made its first flyby early in 2008 and its second this past month on 6 October. After a third flyby next year, MESSENGER will settle into orbit in 2011 for a planned, year-long mission of observations and experiments. Remarkably, this is only the second spacecraft to journey to Mercury: Mariner 10 made three passes in 1974-5. Even partway through MESSENGER’s mission, unexplored regions of the planet have now or will be soon been imaged in unprecedented detail, and planetary scientists are poring over the images.

Mercury is obviously battered by impacts like the Moon, yet it is visibly distinct. Mercury has no maria (large and dark, flat and relatively uncratered lava basins) or distinct mountain ranges, although volcanoes ring impact basins. On the other hand, both have fresh-looking impact craters and ejecta rays, compare our Moon's Tycho with Mercury's Kuiper, both presenting a navel orange appearance. Compare these two rugged bodies for yourself: what differences and similarities do you see?

The crescent Moon slipped past Mercury during the last weekend of October and for the rest of the year, Venus and Jupiter stand as evening stars. Watch how this pair draws to within 2 degrees by the end of November. The crescent Moon will sweep past the pair on the first of December. And don’t miss this treat for the New Year: the crescent Moon again joins the evening stars, Venus and Jupiter and this time, Mercury, in a lovely celestial conjunction for the drive home around the holidays.

For more information on celestial events and sights, see Guy Ottewell’s Astronomical Calendar for 2008 and 2009 as well as Francis Reddy and Greg Walz-Chojnaki’s Celestial Delights. Moondark is written by Douglas C. Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 25 October 2008. Text on this web page is free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses.



The Moon ...



or Mercury?



Source: NASA's MESSENGER Mission to Mercury site