Overview
Four hundred years ago, our Moon and Mercury would have seemed an odd pair to be studied together. The Moon is our constant companion, orbiting from the daytime to the night time sky once every month, its largest surface details visible to humans throughout history. Mercury, on the other hand, is the most difficult of the naked eye planets to observe. It remains always close to the Sun, alternating from evening star to morning star, poking above the horizon to a maximum altitude of anywhere from 28 to just 17 degrees before heading swiftly back towards the Sun. Before the telescope, this is all we knew.
However, since the telescope was invented notable similarities have slowly stacked up. In 1631, Mercury was seen to transit in front of the Sun, something like the Moon does during a solar eclipse—only blocking a lot less of its light. In 1639, Mercury was found to run through a similar series of phases as the Moon. As with Galileo’s observations in the case of Venus, this showed that Mercury orbits the Sun, adding even more weight to the heliocentric hypothesis.
While the Moon’s cratered, rocky surface was also revealed through the telescope’s invention, Mercury’s surface remained hidden for a long time due to its proximity to the Sun and the associated difficulty of observing it. In the meantime, however, other similarities were discovered. For instance, astronomers found that Mercury, much like the Moon, does not spin freely due to tidal forces that have locked its rotation into a resonant pattern with its orbit. Then in 1974, the first close-up pictures of Mercury showed an airless, rocky, cratered planet that indeed does closely resemble the Moon in overall appearance. Perhaps one of the most interesting coincidences is that both the Moon and Mercury contain enormous amounts of water ice that remains trapped in craters at their poles that never see sunlight.
Over the years, both the Moon and Mercury have revealed some big surprises, and these have shaped and reshaped our understanding of each of them. For example: discovering the Moon’s mare basalts, and discovering Mercury’s large iron core and high density. Later data have revealed Mercury’s magnetic field, volatile-rich surface composition, and evidence of volcanic plains extending much more recently than once thought. In this module, you will explore what we have learned about the Moon and Mercury through the observations that we make, beginning with an explanation of the phases and eclipses we observe, and then moving on to the discoveries that have come from sending spacecraft to study them.
Learning Objectives
When you have finished this module, you should be able to do the following:
- Explain the Moon’s phases and the cause of solar and lunar eclipses.
- Compare the Moon and Mercury in terms of orbits, observational aspects, atmosphere, temperature, impact craters, and evidence of water ice in their polar regions.
- Summarise the results of past and ongoing missions that have explored the Moon and Mercury.
Key Terms and Concepts
- cycle of lunar phases (new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent, new moon)
- sidereal period/synodic period
- solar eclipses (total, partial, annular)
- lunar eclipses (total, partial, penumbral)
- line of nodes
- eclipse season
- umbra/penumbra
- transit
- synchronous rotation
- maria (singular, mare)
- Apollo missions
- late heavy bombardment
- large-impact hypothesis
- Mariner 10 mission
- gravity assist
- MESSENGER mission
- BepiColombo mission (ESA–JAXA)
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