The Planet Mercury
In The Telescope

from Hermograph Press

In our last orbit, we told you how to locate the planet in the sky. Now we tell you what to expect in a telescope. Watch out! It's tricky but, then again, the planet IS named for the ancient god of thieves! (See All Things Mercury's Hermes description).

Note: This page is always under construction.


Mercury goes through phases. These were first noted by Johannes Hevelius (1611-1687), a noted lunar mapmaker. It takes a telescope to view these; even an inexpensive Christmas telescope can view them, though the better the optics, the easier this will be.

The reason Mercury has phases is that it is always closer to the Sun than we are; it is never behind us. So we will sometimes see the fully lit hemisphere of Mercury, sometimes the fully dark hemisphere, or usually some portion of both. When Mercury is between us and the Sun (inferior conjunction), its sunlit side faces away from Earth. We see only the dark side (and then, we only see that when we observe a transit of Mercury across the Sun's face). At superior conjunction, the Sun is between us and Mercury and we could see the day side of the planet, if the solar glare wasn't in the way. When Mercury is to the side of the sun, we see fractions of both day and night sides of the planet.

The two most interesting phases to observe are the half-moon shapes and the crescent shapes. The half-moon times occur at the Greatest Elongation dates. At that time there is a straight edge visible in a telescope, marking the day/night boundary, while the side facing the sun is a filled semi-circle. During the evening appearances of Mercury (Eastern Elongations), the planet starts to move back into the solar glare after it has reached its maximum visible distance from the Sun. It also moves closer to us and gets larger in apparent size but also shows more and more of its dark side to us. Since the planet is a sphere, what we see of the lit side has a crescent shape. Maximum visible brightness (greatest brilliancy) usually happens within a week before or after greatest elongation, a function of the angular size of the planet, and the phase.

Following solar conjunction, everything happens in reverse. Mercury appears in the dawn sky as a crescent, expands to a half-moon shape but shrinks in apparent diameter, brightens to a peak (usually) somewhere within a week of the half-moon phase of Greatest Western Elongation day.

The Surface

"Mercury seen in Swedish Solar Vacuum Telescope" In a telescope the planet appears slightly pinkish but this is mostly due to being viewed low in our atmosphere, just as the Sun. In the steadiest conditions, viewed in the largest telescopes, some faint dusky markings sometimes appear. The contrast between them and the surrounding surface is poor and these are difficult to map. Johann Schroter, a German astronomer (1745-1816), observed what he thought were high mountains because they were bright spots in the dark hemisphere of Mercury, separated from the rest of the sun-lit side. He also observed some dark markings and derived a rotation period just over 24 hours. A British astronomer W. Denning determined a 25-hour rotation. Percival Lowell, the American astronomer who found canals on Mars (or so he thought), also drew canals on Mercury.
In the later 1800's, a certain number of features were seen on a regular basis. In fact, the Italian astronomer Giovanni V. Schiapareilli (1835-1910) drew what passed as the first definitive map of the planet, including a giant "Figure 5" that dominated maps of the planet well into the 20th century. These regularly seen features lead to the conclusion that Mercury was locked in a tidal embrace with the Sun, one face always facing the sunshine, the other always facing away, just like our Moon always facing Earth. The conclusion, Mercury had to rotate at the same speed at which it orbited the Sun, 88 days.

Noted sharp-eyed observer Eugenios M. Antoniadi's 1934 map laid the basis for a naming system. A dark area was called Solitudo (for "Wilderness"). With one exception, every named feature had something to do with gods, people or places associated with ancient Greek and Egyptian mythologies of Hermes. A couple of elongated streaks that appeared to be valleys were titled Vallis.

However, Mercury was playing a practical joke on astronomers. Several factors caused earthbound astronomers to always see the same features over and over. There is a tidal connection between Mercury and the Sun. But it is not 1:1 but a 1.5:1. Mercury actually rotates 1 and a half times for every "year" of Mercury; Mercury's day equals about 59 Earth days. And, there is a "synchronicity" factor with Earth's orbital period, and the earth's axis tilt doesn't help either. [Click here to take a side trip on the story of the Drawing the Right Surface Maps of Mercury.] We now know that Mercury rotates in 58.6 days and that we SHOULD see all sides of Mercury from Earth. This was determined in 1965 when radar, sent to the planet and reflected back to Earth, had its return frequency changed, but not by the amount of change expected for a slower 88-day rotation period. (Think how the horn of a slow car changes little in frequency as it passes you but the horn of a faster car changes quite dramatically.)

The few dusky markings spotted on Mercury from Earth don't correlate well with features spotted by Mariner 10. Nevertheless they can be plotted on top of the various surface features seen by that probe, and have been so, and made part of the current nomenclature system of the planet.

PREVIOUS ORBIT (2/5): Mercury in the Sky | NEXT ORBIT (4/5): Closeups of Mercury - Mariner 10 and Messenger