Mercury best before dawn on July 20
the planet Mercury
(Image credit: NASA)
Thursday, July 19, 2007.
Tomorrow, Mercury will swing farthest from the sun on our sky’s dome for this current predawn apparition.
This is what’s called a greatest western elongation of Mercury. Because this world is now west of the sun, Mercury precedes the sun into the morning sky. It rises in the east less than an hour and a half before sunrise tomorrow, so you have to catch this world shortly after it rises – but before it’s obscured in the predawn glare.
Mercury, the innermost planet of the solar system, never strays far from the sun as seen in Earth’s sky. It stays close to our daytime star, as if it’s on a short tether. Mercury swings from its farthest point west to its farthest point east of the sun. It yo–yos from morning to evening sky, appearing in one or the other half a dozen times every year.
As a general rule, when Mercury puts on a good show in one hemisphere, it’s poor in the other. But, around now, very shortly before the sun rises, people from both hemispheres have a decent chance of spotting Mercury near the sunrise point on the eastern horizon.
Mercury is bright, but there are a number of bright stars up before dawn now. Come to the skywatching center for a sky chart.
Our thanks today to Research Corporation: America’s first foundation for science advancement.
Skychart for night of July 19: Observer’s challenge: Mercury at and before dawn
from Earth & Sky’s Skywatching Center.
The best time to see Mercury in the morning sky is when Mercury’s greatest morning elongation occurs in close conjunction with the autumnal equinox (September 23 in the northern hemisphere; March 20 in the southern hemisphere).
On the other hand, the best evening view of Mercury takes place when Mercury’s greatest evening elongation happens near the spring equinox. Mercury’s next greatest evening elongation will be forthcoming on September 29, 2007, about one week after the southern hemisphere’s spring equinox.
For the southern hemisphere, that means a great evening apparition of Mercury in late September and early October. The northern hemisphere misses out, because this evening elongation of Mercury coincides with their autumnal equinox.
Dates for Mercury’s greatest elongations
The planet Mercury by Bill Arnett
Rising time of Mercury – Old Farmer’s Almanac
