The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631 Pierre Gassendi made the first observations of the transit of a planet across the Sun when he [...]

The earliest known recorded observations of Mercury are from the MUL.APIN tablets. These observations were most likely made by an Assyrian astronomer around the 14th century BC. The cuniform name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU4.UD (”the jumping planet”). Babylonian records of Mercury date back to the 1st millennium [...]

Mercury’s apparent magnitude varies between about −2.0—brighter than Sirius—and 5.5.[62] Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun’s glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight. The Hubble Space Telescope cannot observe [...]

For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times, in the same way that the same side of the Moon always faces the Earth. However, radar observations in 1965 proved that the [...]

During the 19th century, French mathematician Le Verrier noticed that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He proposed that another planet might exist in an orbit even closer to the Sun to account for this perturbation. (Other explanations [...]

Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. It takes 88 days to complete an orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the [...]