THE INNER PLANETS MERCURY Venus AND MARS larger than Mercury However Mercury is larger than the dwarf planet Pluto For some 75 years when Pluto was classified as a planet Mercury was considered the second smallest planet Mercury is the densest planet in the solar system followed by Earth if one takes into account the planets internal compression because of gravity Mercury is unusually dense because it is composed of a high per centage of metal The metal is concentrated in a comparatively huge core which accounts for nearly 75 percent of Mercury s diameter AppEARANCE FROM EARTH Mercury can be seen from Earth without a telescope It always appears close to the Sun within about 28 angular degrees For this reason the planet can only be seen near the horizon At certain times of the year it appears as a morning star just before sunrise while at other times it appears as an evening star just after sunset Because Mercury's orbit lies between Earth s orbit and the Sun Mercury displays phases like those of the Moon and the planet Venus These phases can only be seen with the aid of a telescope Mercury sometimes 12
CHARACTERISTICS OF MERCURY O SO Five separate images were combined to show Mercury crossing in front of the Sun during a type of eclipse called a transit on Nov 15 1999 The false color images were taken in ultraviolet light by the Transition Region and Coronal Explorer TRACE satellite in Earth's orbit The time between successive images was about seven minutes NASA TRACE SMEX looks like a crescent to observers on Earth At other times when Mercury and Earth are in different positions more of the sunlight reflected off Mercury can be seen from Earth Mercury appears as DEAN then a half or fuller disk About a dozen times each century Mercury passes directly between Earth and the Sun This event called a transit is a type of eclipse During a transit of Mercury the planet appears as a small black disk against the background of the bright Sun Binoculars 13
THE INNER PLANETS MERCURY Venus AND MARS or a small telescope safely equipped with a solar filter are required to observe a tran sit preferably by projection of the Sun s image onto a white card The next transit of Mercury is in 2016 ORBIT AND SPIN On average Mercury orbits the Sun at a dis tance of nearly 36 million miles 58 million kilometers Like all the planets it travels around the Sun in an elliptical oval shaped orbit Mercury s orbit is the most eccentric or elongated of all the planets Its orbit is also the most tilted The plane of Mercury s orbit is tipped about 7 degrees relative to the ecliptic or the plane of Earth's orbit Mercury completes one orbit around the Sun about every 88 Earth days In other words one year on Mercury lasts some 88 Earth days The planet was named after the ancient Roman god Mercury the counterpart of the ancient Greek god Hermes Like Hermes the swift footed messenger of the gods the planet Mercury is known for the speed with which it moves across the sky The planet circles the Sun at an average rate of about 30 miles 48 kilometers per second the fastest of the eight planets 14
CHARACTERISTICS OF MERCURY Numerous craters and impact basins mark the surface of Mercury An image taken by a space probe called the Messenger during its first flyby in January 2008 shows part of the planet NASA Johns Hopkins University Applied Physics Laboratory Carnegie Institution of Washington ENTS Although Mercury moves along its orbit very quickly it spins slowly It takes almost 59 Earth days to complete one rotation about its axis Mercury rotates on its axis only three times for every two revolutions it makes around the Sun The combination of a slow spin and a fast orbit leads to an 15
