Five of the solar system’s eight or nine planets (depending on how you count little Pluto) are visible among the stars in the night time sky. From closest to the sun out: Mercury, Venus, Mars, Jupiter and Saturn. Of these naked-eye planets, Mercury is the most difficult to see.
Mercury is tough to spot for two reasons. First, because it is the closest planet to the sun, and second, because it orbits the sun faster than all other planets.
Because Mercury is so close to the sun, it always appears near the sun in the sky from our earthly vantage point. The only times we can catch a glimpse of Mercury is when it is situated along its orbit some ways out to one side of the sun. And then we have to look either to the west, shortly after sunset, or to the east, shortly before sunrise, depending on which side of the sun Mercury is set off to.
Wednesday next week, Oct. 22, Mercury reaches a configuration astronomers call greatest elongation. At its greatest elongation, a planet appears as far from the sun as it ever gets. For us on earth, only two planets have greatest elongations: the two inner planets, Mercury and Venus.
The horizon at dawn. Jupiter is the yellow symbol, Mercury the greenish symbol. The map was created using John Walker's 'Your Sky.' Click the image to visit this website.
In fact, astronomer’s recognize two greatest elongations. Greatest eastern elongation is when a planet is as far to the east of the sun as it ever gets; greatest western elongation is when a planet is as far to the west of the sun as it ever gets. But don’t get caught looking the wrong way: to see a planet at it’s greatest western elongation, for example, look east shortly before sunrise.
Mercury is at greatest western elongation on Wednesday. Look east about a half hour before sunrise Wednesday, and for several days before and after, for a bright star low on the horizon. That will be Mercury.
But after next week, Mercury will quickly sink in the east back toward the sun, to again be lost in the sun’s glare.
Mercury is the fastest planet to orbit the sun, zipping around in just 88 days. So you have to look fast to catch a glimpse.
The moon and the planet Saturn can help you find Mercury next week. Saturn is about 20 degrees above the eastern horizon at dawn, and is easily the brightest object in the area. The moon next week is in its waning crescent phase, and will thin and sink to the east as the week wears on. Mercury will be low on the horizon below the moon and Saturn.
Mercury is the target for a space probe launched by NASA back in 2003. The space probe is called MErcury Surface, Space ENvironment, GEochemistry, and Ranging — or MESSENGER for short. MESSENGER is presently on a complex trajectory around the inner solar system bleeding speed to eventually orbit Mercury. In fact, two weeks ago, on Oct. 6, the little space probe zipped passed Mercury at an altitude of only 200 km, returning the most detailed pictures of a side of Mercury we can’t see from earth.
An image of Mercury taken recently by the Messenger space craft, courtesy the Messenger team at NASA. Click on the image to see a larger version.
You can see the pictures MESSENGER recently took and follow the mission’s progress at http://messenger.jhuapl.edu. MESSENGER is scheduled to blow past Mercury again on Sept. 29, 2009 and to arrive and go into orbit on March 18, 2011.
Pictures on the Internet are great, but reality is better. Take advantage of the late sunrises we have this time of year and get out for a look with your own two eyes.
This column originally appeared in the Grand Haven Tribune on 17 October 2008.
