Mercury in Transit Across the Sun

See the little dot on this pic? That's the silhouette of the planet Mercury as its orbit took it directly between the sun and the earth earlier today. The disparity in size becomes even greater when one realizes that the sun was approximately 30 million miles further away from the earth, than Mercury was at the time.




Doing research on the planet has always been difficult for astronomers, due to Mercury's small size and proximity to the sun. This is an old map of Mercury drawn by the Italian astronomer Eugene Antoniadi, early in the 20th century. He drew the map while peering at the planet through a telescope. At that time, they thought that Mercury always kept one face to the sun at all times, just as the moon always keeps one face to the earth.

Subsequent research found that that was not true. The planet rotates on its axis three times for every two revolutions around the sun. Therefore, this map was not of much use!

In 1975, the interplanetary probe Mariner 10 flew by Mercury and took some photos. This is what it really looks like. Not too different from the moon.