Mercury is the smallest planet in our solar system (assuming you don’t count Pluto). It’s only slightly larger than our own Moon, smaller than the moons Ganymede, Callisto, or Titan, and quite a bit smaller than Mars. Mercury has no significant atmosphere, and its cratered surface looks much like our Moon, albeit free of the large maria (seas) or mountain ranges. Mercury is much denser than the Moon or Mars, however, due to its unusually large metal core. This is likely because Mercury is what’s left of a bigger body that was hit by something billions of years ago.
Mercury is probably the least frequently observed of all eight planets in our Solar System, despite being visible to the naked eye. Many astronomers never see it or go years before doing so. Mercury has only been visited by two orbiting space probes (with a third en route, ESA’s BepiColombo), and no spacecraft has yet landed on its surface. Mercury is often left out of astronomy books, guides, and general observations because it hasn’t been studied much and is hard to see. It also looks a lot like the Moon, which makes it hard to notice.
Mercury is actually pretty easy to spot, provided you know when and where to look, and it shows its phases in a telescope much like Venus. As with Venus and the ice giant planets, the fun of observing Mercury lies more in what it is than in the actual details. However, Mercury is definitely a must-see, and at the very least, it gives you something to observe while you’re waiting for twilight to end and observe faint fuzzies.
When to Observe Mercury – It’s All About Timing
Since it is the closest planet to the Sun, Mercury is never far from it in the sky. Its greatest distance from the Sun is just 47% of the distance between the Sun and the Earth, and it never strays more than about 28 degrees from the Sun in the sky. Thus, Mercury will be practically hitting the horizon when astronomical twilight ends in the evening or just clearing it in the morning, even during its best apparitions, and it is never particularly high up in the sky to begin with. As such, you will pretty much never be viewing Mercury against a black sky, and it is always low on the horizon, both of which are really sub-optimal for viewing fine detail.
You can look at Mercury during the day if you shield your telescope from direct sunlight and use something to help you find it. This allows you to view Mercury when it is at its highest and least disturbed by atmospheric turbulence (though low in contrast against the blue sky). However, doing so is difficult, and observing in direct sunlight is, of course, dangerous. Never use your telescope to aim at daytime objects anywhere near the direction of the Sun. Only try observing planets in the daytime if you can get into the shade; don’t use an open or truss tube telescope in the daytime, and do not aim your telescope at the Sun without a specialized safe solar filter.
The easiest way to observe Mercury, however, is to simply wait for it to be as far from the Sun and as high in the sky as possible. This occurs for a handful of days surrounding its greatest elongation in the sky from the sun. During these short windows, Mercury will usually lie between 15 and 28 degrees from the Sun in the sky and thus be fairly conspicuous in the sky either shortly after sunset or before sunrise. Each greatest elongation in the morning/evening is spaced out by about 116 days (which is referred to as the synodic period), so one or the other occurs about once every two months. Mercury rises rapidly in the evening sky as it approaches us from the far side of the Sun, then sinks again as it slips between the Earth and Sun over a period of just a handful of days. Then it reappears gradually in the dawn sky a few weeks later before swinging back behind the Sun. Generally, you’ll have the best luck if you look for Mercury within 3-5 days around the date of greatest elongation
Mercury’s greatest elongation west (for best morning visibility) will next occur on:
- January 30, 2023
- May 29, 2023
- September 22, 2023
- January 12, 2024
- May 9, 2024
- September 5, 2024
- December 25, 2024
Mercury’s greatest elongation east ( for best evening visibility) will next occur on:
- April 11, 2023
- August 10, 2023
- December 4, 2023
- March 24, 2024
- July 22, 2024
- November 16, 2024
Finding Mercury
Mercury is actually one of the brightest objects in the night sky at times. At its brightest, Mercury reaches a magnitude of -1.6, beating out even the brightest stars and being similar to Mars, Jupiter, the International Space Station, and the Tiangong space station at their brightest. However, due to the fact that Mercury is pretty much never visible in total darkness, its brilliance is usually washed out by the bright twilight sky. Mercury appears yellow, orange, or pink due to its typically low altitude, just like the rising or setting Sun does.
Observing Mercury through a Telescope
Once you find Mercury in your telescope, the most immediate thing you’re probably going to see – if you can – is its phase. Mercury’s disk varies from around 5 arc seconds to 13 arc seconds, making it smaller than the disk of Saturn (15–20 arc seconds) and often smaller than that of Mars. To see the phase, you need a magnification of 50x or more, and a 6-8″ or larger telescope in good seeing conditions will help you see Mercury better.
Mercury lacks any kind of high-contrast, dark relief markings that our Moon, Mars, or Jupiter’s moon Gaymede have, with the most prominent features being a handful of ray craters like those on the Moon. However, their small size and the low contrast of the twilight sky, combined with less-than-optimal seeing conditions near the horizon, mean that even with an 8” or bigger telescope, it’s unlikely you’ll see anything, even with practice and experience. Astrophotography with a high-speed camera and stacking may bring out the faint relief markings of the rays and some craters, but don’t get your expectations up. Sometimes just seeing Mercury’s phases can be an achievement in and of itself.
Transits of Mercury
Like Venus, Mercury transits in front of the Sun when it passes directly between the Sun and the Earth every so often. This happens in May or November once or twice per decade when Earth’s and Mercury’s orbits perfectly line up. The next pair of these will be on November 13, 2032, and November 7, 2039. You can observe these transits safely with a dedicated solar telescope, or a Mercury’s disk can be seen at fairly low power (30x or above) during a transit as a small black disk, which may be easy to confuse with sunspots if there are any nearby. Mercury’s silhouetted disk takes a few hours in total to cross the entire disk of the Sun. It’s a spectacular sight that is definitely worth going out of your way to view, though thankfully, transits can be seen anywhere on Earth that is experiencing daylight at any point in the duration.
