Before the Mariner 10 mission of Mercury, it was very difficult to see any markings on the surface of the planet from Earth. This image shows a view of Mercury obtained from a telescope on Earth. The first attempts to find out the day length of the planet found an 88 earth-day rotation period, equal to the orbital period, or year length. It was only in the 1960s, when a radar technique allowed the rotation rate to be determined, we found that Mercury spins on its axis every 59 Earth days. But the length of a day on Mercury is about three times this. To find out why, click the link below.

Before the Mariner 10 mission of Mercury, it was very difficult to see any markings on the surface of the planet from Earth. This image shows a view of Mercury obtained from a telescope on Earth. The first attempts to find out the day length of the planet found an 88 earth-day rotation period, equal to the orbital period, or year length. It was only in the 1960s, when a radar technique allowed the rotation rate to be determined, we found that Mercury spins on its axis every 59 Earth days. But the length of a day on Mercury is about three times this. To find out why, click the link below.

Prior to the Mariner 10 mission of Mercury, it was very difficult to see any markings on the surface of the planet from Earth. This image shows a view of Mercury obtained from a telescope on Earth. Initial attempts to find out the rotation rate of the planet found an 88 earth-day rotation period, equal to the orbital period, or year length. It was only in the 1960s, when a radar technique allowed the rotation rate to be determined, we found that Mercury spins on its axis every 59 Earth days. But the length of a day on Mercury is about three times this. To find out why, click the link below.