Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars
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 saw a transit of Mercury predicted by Johannes Kepler. In 1639 Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.
A very rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory. The next occultation of Mercury by Venus will be on December 3, 2133.
The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800 Johann Schröter made observations of surface features, claiming to have observed 20 km high mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°. In the 1880s Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury’s rotational period was 88 days, the same as its orbital period due to tidal locking.This phenomenon is known as synchronous rotation and is also shown by Earth’s Moon. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations. Many of the planet's surface features, particularly the albedo features, take their names from Antoniadi's map.
In June 1962 Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences lead by Vladimir Kotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the planet. Three years later radar observations by Americans Gordon Pettengill and R. Dyce using 300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet’s rotational period was about 59 days.The theory that Mercury’s rotation was synchronous became widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.
Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury’s orbital period, and proposed that a different form of tidal locking had occurred in which the planet’s orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.Data from Mariner 10 subsequently confirmed this view. The 3:2 resonance results from Mercury's eccentric orbit, as the Sun raises higher tides on the planet at perihelion which, combined with the planet's high velocity then, make the planet spin faster. This also means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every second orbit and recorded them, but regarded those seen in the meantime, when Mercury's other face was toward the Sun, as spurious.
Ground-based observations did not shed much further light on the innermost planet, and it was not until space probes visited Mercury that many of its most fundamental properties became known. However, recent technological advances have led to improved ground-based observations. In 2000, high-resolution lucky imaging observations were conducted by the Mount Wilson Observatory 1.5 meter Hale telescope. They provided the first views that resolved some surface features on the parts of Mercury which were not imaged in the Mariner missionsLater imaging has shown evidence of a huge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It has informally been dubbed the Skinakas Basin.Most of the planet has been mapped by the Arecibo radar telescope, with 5 km resolution, including polar deposits in shadowed craters of what may be water ice.
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