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, which is probably the result of Mercury being the remnant of a larger body that was impacted billions of years ago. Mercury was named by the ancient Romans after the swift-footed Roman messenger god, Mercurius, due to its swift motion in the sky.
Mercury is probably the most seldom observed of all eight planets in our Solar System, despite being visible to the naked eye. Many astronomers never see it or wait 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. Due to the lack of exploration and the difficulty of observing it, along with its resemblance in appearance to the Moon, Mercury is often ignored in astronomy texts, guides, and by observers at large.
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 gives you something to observe while you’re waiting for twilight to end and observing 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 suboptimal for viewing fine detail.
It’s possible to observe Mercury during the day by shielding your telescope from direct sunlight and using some sort of aid to locate it in your telescope. 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 in 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, which 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, sinks again as it slips between the Earth and Sun over a period of just a handful of days, and then 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.
- 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:
- 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 magnitude -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
The first known observations of the phases of Mercury were made by Italian astronomer Giovanni Battista Zupi in the early 17th century. Although his telescope was not powerful enough to see Mercury in detail, it could discern the planet’s changing phases, which earlier astronomers like Galileo could not do with their telescopes. These observations, along with those of Galileo and the phases of Venus a few decades prior, provided early evidence for the heliocentric model of the Solar System, where planets orbit the Sun, not the Earth.
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. A magnification of 50x or more is required to see the phase, and you’ll have better luck resolving Mercury with a 6-8” or larger telescope under good seeing conditions.
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.
Throughout the centuries, many astronomers have claimed to have observed details on the surface of Mercury, but they are usually incorrect. For instance, in the 1880s, Italian astronomer Giovanni Schiaparelli reported seeing various features, including what he called “dusky streaks” crisscrossing the planet. These may have been blood vessels in Schiaparelli’s own eyeball, similar to the “canals” and “spokes” other astronomers at the time claimed to see on Mars and Venus when they pushed the limits of their (often fairly modest) telescopes too far and ended up viewing internal reflections of their eyes instead.
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 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.
Conjunctions and Occultations of Mercury
Mercury’s position closest to the Sun in our Solar System and its extremely high speed result in frequent conjunctions with other planets. However, due to its closeness to the Sun, these conjunctions often take place near the horizon around sunrise or sunset. Occasionally, these are close conjunctions where both Mercury and another planet will fit in the same telescopic field of view. For instance, on January 27, 2024, it will be very close to Mars in the dawn sky, and on June 4, 2024, it passes by Jupiter in the evening; these events are very low in the sky and may be impossible to observe for most people.
Mercury also partakes in frequent lunar occultations and conjunctions, where the planet appears very close to our Moon or is blocked by it altogether. Occultations by the Moon happen roughly once per year (the next one being in 2025) but are frequently unobservable, either appearing too close to the horizon or more widely separated (albeit still in close conjunction) for most viewers.
Mercury goes into superior conjunction with the Sun every 116 days and inferior conjunction every 116 days (its synodic period, also the time between evening/morning apparitions) as well, making it unobservable due to its position on the far side of the Sun from Earth or too close to the Sun, respectively. During these times, Mercury is hidden in the Sun’s glare and is at its furthest or closest to us.
Color Filters for Observing Mercury
Filters are not particularly useful on Mercury unless you happen to be observing it in the daytime, in which case an orange #21 filter may help boost contrast somewhat on any visible detail.
Physical Parameters of Mercury
Compared to Earth, Mercury is a small planet, with a mass of only about 5.5% of Earth’s. Its surface gravity is similarly lower, at about 38% of Earth’s surface gravity. This means that if you weighed 100 pounds on Earth, you’d weigh only 38 pounds on Mercury. Compared to the Moon, Mercury is larger and has more than double the mass, resulting in a surface gravity about twice as strong. Mars, on the other hand, is larger than Mercury but less dense, resulting in a surface gravity similar to Mercury’s.
Unlike many other planets in the Solar System, Mercury has no moons, no ring system, and no substantial atmosphere to form clouds. Its sky is always black and filled with stars, even during the day. Mercury has a weak magnetic field, about 1% as strong as Earth’s. The field is thought to be generated by a dynamo process involving the planet’s molten core, but its existence is still somewhat puzzling given Mercury’s slow rotation.
Unlike most planets, which have nearly circular orbits, Mercury’s orbit is highly elliptical. This means the distance between Mercury and the Sun varies widely throughout the Mercury year, contributing to the planet’s extreme temperature variations. This also affects its placement in the sky; Mercury is best viewed at its furthest point from the Sun (aphelion), while it is also at its greatest apparent angle from the Sun as viewed from Earth, but these two parameters rarely coincide.
Mercury exhibits a unique phenomenon known as a spin-orbit resonance, wherein its rotational period is linked to its orbital period around the Sun. The planet completes exactly three rotations on its axis for every two revolutions it makes around the Sun. This 3:2 resonance led to a fascinating situation for the Mariner 10 mission. As Mariner 10 approached Mercury, each encounter occurred at almost the same point in Mercury’s solar day, meaning the spacecraft only managed to photograph the same side of the planet under varying conditions of sunlight.
Initially, it was believed that Mercury was in a 1:1 tidal lock, meaning it rotated on its axis once for every orbit around the Sun, like the Moon has the same side facing Earth. This would imply that one side of the planet was perpetually facing the Sun, while the other side was always in darkness. However, radar observations in the 1960s revealed that Mercury’s actual spin-orbit resonance is 3:2, not 1:1. Therefore, while the Sun appears to move very slowly in Mercury’s sky, all parts of the planet’s surface experience periods of daylight and darkness.
Mercury orbits the Sun in just 88 Earth days, the shortest of any planet, but its day-night cycle (synodic period) is much longer due to its spin-orbit resonance. One solar day—the time from noon to noon on the planet’s surface—lasts 176 Earth days, or 2 Mercury years.
