The ‘Evening Star’ Points To Mercury

Even though it’s often brighter than the brightest stars, Mercury is always a challenge to spot. Being the planet nearest the Sun, its elusiveness derives from its proximity to our blindingly bright star. While we might tend to forget it, the daytime sky is just as full of stars as the nighttime sky. We just don’t see them since they are obscured by the glare of the Sun. Likewise with Mercury. Since it orbits close to the Sun, it is up virtually all day every day, but it, too, is hidden by the Sun’s glare.

Even though it’s often brighter than the brightest stars, Mercury is always a challenge to spot. Being the planet nearest the Sun, its elusiveness derives from its proximity to our blindingly bright star.

While we might tend to forget it, the daytime sky is just as full of stars as the nighttime sky. We just don’t see them since they are obscured by the glare of the Sun. Likewise with Mercury. Since it orbits close to the Sun, it is up virtually all day every day, but it, too, is hidden by the Sun’s glare.

During about half of Mercury’s orbit, it is either behind the Sun or between Earth and Sun, and thus too near the Sun for us to see. But there are two windows of opportunity in its orbit when it can be seen, even if briefly and with some effort.

Three to four times each year Mercury can be seen, usually for a couple of weeks, low in the east in the morning as dawn breaks, and likewise, three to four times low in the west in the evening at dusk.

Since Mercury’s orbit is highly elliptical (it deviates notably from a perfect circle), some of its appearances are better than others. When it is farther from the Sun than usual, it can be seen sooner before sunrise or longer after sunset, and can be spotted a bit farther from the horizon.

It is now having its best appearance for this year, and to our good fortune, it is near “evening star” Venus, making it easier to spot. The two are low in the west at dusk and remain above the horizon more than an hour after sunset. As they are low, you’ll need a viewing site with a clear view of the western horizon.

Begin looking soon after sundown. The much brighter Venus will appear first; then a little later Mercury will pop into view to Venus’ lower right. Binoculars can help spot Mercury sooner although as the sky darkens, it becomes visible to naked eyes.

The two are at their nearest April 4 with Mercury six moonwidths to Venus’ lower right. Mercury is at its farthest from the setting Sun April 8.

A week later a guest joins the pair for a special show. April 15, a very thin the crescent Moon is three moonwidths to the upper right of Mercury with Venus farther to their upper left. Then the next evening, the Moon is above the two planets and just below the lovely Pleiades (Seven Sisters) star cluster.

While this is Mercury’s best appearance for 2010, its best morning appearance comes in mid September, although Venus won’t be nearby.

Erratum: Thanks to teachers Steve Salvesen and John Herbert and their Lake Waco Montessori 5th graders for noting an error in last column’s diagram. The Moon, of course, orbits the Earth.

• Sky Calendar.

* Apr. 6 Tue.: The Moon is at 3rd quarter.

* 11 Sun. morning: The crescent Moon is above Jupiter low in the east at dawn, and to the planet’s left the next morning.

* 14 Wed.: The Moon is new.

* 17 Sat. evening: Mars is a moonwidth from the Beehive star cluster – best seen in binoculars or a scope a low power.

* 21 Wed.: The 1st quarter Moon is below Mars.

* 22 Thu. morning: Lyrid meteor shower peaks – best after the Moon sets at 3 a.m.

• Naked-eye Planets. (The Sun, Moon, and planets rise in the east and set in the west due to Earth’s west-to-east rotation on its axis.) Evening: Venus and Mercury are low in the west with Saturn well up in the southeast and Mars high overhead. Morning: An hour before sunrise, Saturn is setting in the west as Jupiter is rising in the east.

• Star Party. The Central Texas Astronomical Society’s free monthly star party is Apriil 17 at the Lake Waco Wetlands beginning at 8 p.m., weather permitting. For directions see my Web site.

Stargazer appears every other week. Paul Derrick is an amateur astronomer who lives in Waco. Contact him at 918 N. 30th, Waco, 76707, (254) 753-6920 or paulderrickwaco@aol.com. See the Stargazer Web site at <stargazerpaul.com>.