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Astronomy 2021: Closer to Home – The Solar System
“Uncle Bob”


Mercury speeds around the Sun every 88 days, traveling through space at nearly 29 miles per second, faster than any other planet.

By contrast Mercury spins slowly on its axis and completes one rotation every 59 Earth days. Its spin is so slow that earlier observers thought that the planet had constant faces toward and away from the Sun, as does our Moon in relation to us. One Mercury solar day (one full day-night cycle) equals 176 Earth days—just over two Mercury years.

Mercury has a more elliptical orbit than other planets and this causes some irregular movements of the Sun in its sky.

A long day on Mercury is described here.

In spite of its closeness to the Sun, a Mercury transit, that is, Mercury crossing the Sun from our point of view, is a rare event, happening just 13 times per century on average. As fast as the planet is in its orbit, the transit duration is a lengthy 7.5 hours.

Watch a Mercury transit.

Mercury's Surface
Moon's Far Side


Mercury's surface resembles that of Earth's Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. Craters and features on Mercury are named after famous deceased artists, musicians or authors, including children's author Dr. Seuss and dance pioneer Alvin Ailey.

See a 3-D model.

The first rock away from the Sun suffered a major trauma earlier in its life. The Caloris Basin was created by a tremendous impact that we think reduced the planet’s size and even had physical effects on the planet’s side opposite the crash.

Mercury may have water ice at its north and south poles inside deep craters, but only in regions of permanent shadow, where it could be cold enough to preserve water ice despite the high temperatures on sunlit parts of the planet.

Mercury's environment is not conducive to life as we know it. The temperatures and solar radiation are most likely too extreme to be survivable given current technology.

Let’s make the attempt, anyway. Spend a day on Mercury.

Better protections from radiation may come about. But to survive on Mercury would require advanced, detailed knowledge of local terrain and climate, and careful planning of movement to keep in moderate zones. Can you imagine a Mercurial crater at the edge of sunlight and darkness which has one rim boiling hot and the opposite rim far below freezing?

Next month: The King of the Rings, Saturn.

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