Mercury (planet) - Observation

Observation

Mercury’s apparent magnitude varies between −2.6 (brighter than the brightest star Sirius) and about +5.7 (approximating the theoretical limit of naked-eye visibility). The extremes occur when Mercury is close to the Sun in the sky. 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.

Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.

Like the Moon and Venus, Mercury exhibits phases as seen from Earth. It is "new" at inferior conjunction and "full" at superior conjunction. The planet is rendered invisible from Earth on both of these occasions because of its relative nearness to the Sun.

Mercury is technically brightest as seen from Earth when it is at a full phase. Although the planet is farthest away from Earth when it is full the greater illuminated area that is visible and the opposition brightness surge more than compensates for the distance. The opposite is true for Venus, which appears brightest when it is a crescent, because it is much closer to Earth than when gibbous.

Nonetheless, the brightest (full phase) appearance of Mercury is an essentially impossible time for practical observation, because of the extreme proximity of the Sun. Mercury is best observed at the first and last quarter, although they are phases of lesser brightness. The first and last quarter phases occur at greatest elongation east and west, respectively. At both of these times Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion. At greatest elongation west, Mercury rises at its earliest before the Sun, and at greatest elongation east, it sets at its latest after the Sun.

At tropical and subtropical latitudes, Mercury is more easily seen than at higher latitudes. This is the result of two effects: (i) the Sun ascends above the horizon more steeply at sunrise and descends more steeply at sunset, so the twilight period is shorter, and (ii) at the right times of year, the ecliptic intersects the horizon at a very steep angle, meaning that Mercury can be relatively high (altitude up to 28°) in a fully dark sky. Such conditions can exist, for instance, after sunset near the Spring Equinox, in March/April for the southern USA and in September/October for South Africa and Australasia. Conversely, pre-sunrise viewing is easiest near the Autumn Equinox.

At temperate latitudes, Mercury is more often easily visible from Earth’s Southern Hemisphere than from its Northern Hemisphere. This is because Mercury's maximum possible elongations west of the Sun always occur when it is early autumn in the Southern Hemisphere, while its maximum possible eastern elongations happen during late winter in the Southern Hemisphere. In both of these cases, the angle Mercury strikes with the ecliptic is maximized, allowing it to rise several hours before the Sun in the former instance and not set until several hours after sundown in the latter in countries located at southern temperate zone latitudes, such as Argentina and South Africa. By contrast, at the major population centers of the northern temperate latitudes, Mercury is never above the horizon of a more-or-less fully dark night sky.

Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of its surface from 1974 to 1975. The second is the MESSENGER spacecraft, which after three Mercury flybys between 2008 and 2009, attained orbit around Mercury on March 17, 2011, to study and map the rest of the planet.

The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures which prevent its pointing too close to the Sun.