The Planets

Mercury

The hardest part about observing Mercury is finding an unobstructed horizon because, although Mercury is bright (it can exceed magnitude 0), it always hugs the horizon either shortly before dawn or shortly after sunset. I recommend looking for it with binoculars.

Mercury's orbit is inside the Earth's and it never gets more than 27 degrees above the horizon, so it always appears as either a morning "star" or an evening "star." Mercury has phases similar to the moon (but it never appears "full") and it repeats them approximately every four months. This is because while Mercury takes about three months (88 Earth days) to complete its orbit around the Sun, the Earth has moved one quarter of the way through its orbit. Mercury then has to complete another third of its orbit (one month) before it "catches up" with the Earth and again returns to the same position as viewed from Earth.

In early January 2005, Venus and Mercury were both pulling away from Earth as they headed behind the Sun. They were visible about 10 degrees above the horizon 25 minutes before sunrise on January 1, 2005. During the next two weeks they get lower in the sky each morning until January 15 when they were about 7 degrees above the horizon 25 minutes before sunrise. This animation shows their positions at 7:00 AM. Notice that there was a conjunction on January 13.

Updated on 2005-03-03