Mercury is probably the most seldom observed of all eight planets in our Solar System, despite being visible to the naked eye. For a long time, I hadn’t seen it myself—and I know many astronomers who’ve waited years before doing so.
Due to the lack of exploration to Mercury by space probes along with its resemblance in appearance to the Moon, Mercury is often ignored in astronomy texts, guides, and by observers at large.
But 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 or the ice giant planets (Uranus and Neptune), the fun of observing Mercury lies more in what it is than in the actual visual details. Still, I think Mercury is a must-see—and at the very least, it gives me something to observe while I’m waiting for twilight to end for observing faint fuzzies.
- Mercury is the smallest planet in our solar system (assuming we don’t count Pluto). It’s only slightly larger than our own Moon and is smaller than the Jupiter’s largest moon Ganymede and Saturn’s largest Moon, Titan. It is also 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 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.
- 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.
When to Observe Mercury: It’s All About Timing
Since Mercury is the closest planet to the Sun, it never strays far from it in the sky.
Its greatest distance from the Sun is just 47% of the Earth-Sun distance, and it never strays more than about 28 degrees away 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, it is never particularly high up in the sky to begin with. As such, we’ll 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 details.
I’ve experimented with observing Mercury during the day by shielding the telescope from direct sunlight and using some sort of computerized aid to lock onto Mercury. This allows me to view Mercury when it is at its highest and least disturbed by atmospheric turbulence (though low in contrast against the blue day 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.
In practice, I’ve found that the easiest way to observe Mercury 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 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 elongation 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, I’ve had the best luck if I look for Mercury within 3-5 days around the date of greatest elongation.
Mercury’s greatest elongation east (for best evening visibility) will next occur on:
- July 4, 2025
- October 29, 2025
- February 19, 2026
- June 15, 2026
- October 12, 2026
Mercury’s greatest elongation west (for best morning visibility) will next occur on:
- August 19, 2025
- December 7, 2025
- April 3, 2026
- August 2, 2026
- November 20, 2026
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 the actual night sky (total darkness), I usually found its brilliance 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.
Observing Mercury through a Telescope
Once I find Mercury in the telescope, the most immediate thing I usually see—if I can—is its phase. Mercury’s disk size 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. I’ve found that a magnification of 50x or more is required to see the phase, and I’ve had better luck resolving Mercury with a 6-8” or larger aperture telescope under good seeing conditions.
Mercury lacks any kind of high-contrast, dark relief markings that we could see on the Moon, Mars, or Jupiter’s moon Ganymede. Its most prominent features are a handful of ray impact craters like those on the Moon. However, their small size and the low contrast in the twilight sky, combined with less-than-optimal seeing conditions near the horizon, mean that even with an 8” telescope, I usually don’t see anything, even with my level of 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 phenomenon 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 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 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.
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 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.