7 November: Transit of Mercury

On this date in 1631, a transit of Mercury which had been predicted by Johannes Kepler was observed by Pierre Gassendi. Here are ten things you might not know about the planet Mercury.

1. Mercury is the innermost planet in the Solar System and therefore has the shortest year, lasting just 88 days.
2. It's also the smallest planet in the solar system. In fact, it's smaller than a couple of moons - Ganymede and Titan. Mercury has no moons. The diameter of Mercury is 4,878 km (3,031 miles) and its equatorial radius is 2,439.7 kilometres (1,516.0 miles).
3. It is named after the Roman deity Mercury, the messenger to the gods, because it moves across the sky faster than other planets. It has also been called "the jumping planet" (by the Assyrians); Nabu (by the Babylonians, after their messenger god); "the gleaming" or Hermes in ancient Greece. In ancient China, Mercury was known as "the Hour Star"; it's called the "water star" in Modern Chinese, Korean, Japanese and Vietnamese cultures.
4. Mercury has hardly any atmosphere, because it is too small and hot to retain one. What little atmosphere there is consists of hydrogen, Helium, Oxygen, sodium, calcium, Potassium and tiny amounts of other elements.
5. The surface temperature varies a LOT. It can reach 700 K (427 °C; 800 °F) during the day at the equator, but can be as low as −173 °C; −280 °F at night and at the bottom of deep craters at the poles which never get any sunlight. Some observations have suggested there may even be ice there.
6. Geologists estimate that Mercury's core occupies about 42% of its volume - the Earth's core occupies 17% in comparison. Mercury's core has a higher iron content than that of any other major planet in the Solar System and its crust is estimated to be 100–300 km thick. It is the second densest planet in the solar system (the most dense being Earth).
7. Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 of a degree). It was also thought for a long time to be gravitationally locked with the Sun, rather like the Moon is to Earth, so an observer on the sun would only ever see one side of it. However, it does rotate, albeit quite slowly. A day, from sunrise to sunrise, on Mercury is equivalent to 176 Earth Days.
8. Mercury's surface resembles that of Earth's Moon. Surface features are named according to the type of feature. Craters are named for artists, musicians, painters, and authors who have made a significant contribution to their field, and who are dead. Ridges on the planet's surface are named for scientists who have contributed to the study of Mercury. Depressions are named for works of architecture. Mountains are named for the word "hot" in a variety of languages. Plains are named for words for Mercury in various languages. Escarpments are named for ships of scientific expeditions. Valleys are named for radio telescope facilities. The largest known crater on Mercury is Caloris Basin, with a diameter of 1,550 km.
9. Because its orbit is inside that of Earth, Mercury can only be seen from Earth in the morning or the evening, never in the middle of the night. The same is true of Venus. On very rare occasions, Mercury and Venus occult each other. The next time Venus is due to pass in front of Mercury is on December 3, 2133.
10. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus. In astrology, it rules communication, mentality, thinking patterns, rationality and reasoning, adaptability and variability, schooling and education, neighbours, siblings and cousins, transport over short distances, messages and forms of communication such as post, email and telephone, newspapers, journalism and writing, information gathering skills and physical dexterity, the nervous system, the brain, the respiratory system, the thyroid and the sense organs. The day of the week ruled by Mercury is Wednesday which is why, in many European languages, the word for Wednesday is linked to Mercury, eg. mercredi in French. It rules the zodiac signs of Gemini and Virgo.