MERCURY - First Planet
Although one of the brighter objects in the heavens, most people have never seen Mercury. Mercury is difficult to study from the earth because it is so close to the sun. Its maximum angular separation (greatest elongation) is only 28° on either side of the sun. At this favorable position for viewing its phase corresponds to a quarter moon; the full phase occurs at superior conjunction, when Mercury is almost in line with the sun. Swift orbital motion keeps the planet visible low above the horizon for only a few days each year, immediately after sundown or before sunup. It is best seen when it is a morning star during the early part of the year or an evening star during the last half of the year.
Of the terrestrial planets (excluding our moon) Mercury is the least massive, being about 6 percent of the mass of the earth, and smallest in size, being less than twice the size of the moon. However, next to the earth, it has the highest mean density, a point we shall come back to later.
Mercury's rotation period is two-thirds of its orbital period; thus the planet completes three rotations during two orbital revolutions. This synchronization of its rotation and revolution, like that between the earth and the moon, is not accidental; it was apparently set up by the strong tidal pull exerted by the sun. Different parts of the body of Mercury are at slightly different distances from the sun. Thus they experience slightly different accelerations from the sun's gravitational attraction. Such a differential gravitational effect is known as a tidal force, and it is responsible for having slowed the planet's rotation, trapping it so that the ratio of its rotation to revolution period is three to two. As a result, the sun takes 88 days after rising on the eastern horizon to cross Mercury's sky and set on the western horizon; meanwhile the planet completes one orbit of the sun.
