All About Mercury, The Smallest Planet In The Solar System
Mercury is rather bright but still not easy to see: most of the time, it’s close to the Sun in the sky. Learn how to track Mercury’s movements and pick the best time to spot the planet with the astronomical app Sky Tonight!
Contents
- Mercury planet facts
- What’s Mercury like? Size, temperature, composition
- Observing Mercury: a quick guide
- Is Mercury visible tonight?
Mercury events 2025
- October 29: Mercury at greatest eastern (evening) elongation
- November 9: Mercury near Antares
- November 9: Mercury retrograde starts
- November 12: Mercury near Mars
- November 20: Moon near Mercury
- November 20: Mercury at inferior conjunction with the Sun
- November 25: Mercury near Venus
- November 29: Mercury retrograde ends
- December 7: Mercury at greatest western elongation
- December 18: Moon near Mercury
- F.A.Q.
- Planet Mercury: to sum up
Mercury planet facts
- Planet type: terrestrial
- Radius: 2440 km (1516 miles)
- Mass: 3.3011×10^23 kg
- Aphelion: 69.8 million km (43.4 million miles)
- Perihelion: 46.0 million km (28.6 million miles)
- Average distance from the Earth: 77 million km (48 million miles)
- Surface temperature: -173°C to 427°C (−280 °F to 800 °F)
- Solar day length: 176 Earth days
- Sidereal day length: 59 Earth days
- Year length: 88 Earth days
- Age: 4.503 billion years
- Named after: Roman god of commerce
6 facts about Mercury we bet you didn’t know
- There are no seasons on Mercury because its axis has almost no tilt (only 2 degrees).
- Ancient astronomers believed Mercury was two different objects because it could appear in the evening western sky or rise in the morning eastern sky. By the way, they thought the same about Venus.
- Like the Moon and Venus, Mercury has phases, but you can see those only through a telescope.
- Mercury is almost certainly not habitable. Its rough temperatures and closeness to the Sun make the planet too extreme for living organisms.
- Mercury has a tail. Many people know comets have tails. But have you heard about a planet that has one? Mercury has a cometlike tail produced by the solar wind that pushes sodium atoms off the planet’s surface. It’s not easy to see: you’ll have to take a long-exposure photo using a telescope and a special filter.
- In Sanskrit, Mercury is called Budha (not to be confused with Buddha, the founder of Buddhism). The Vedic texts call it Budh planet and refer to it as a deity, the son of Soma (or Chandra, the Moon god) and Tara (wife of Bṛhaspati, the god of Jupiter).
What’s Mercury like? Size, temperature, composition
How big is Mercury?
Mercury is the smallest planet in the Solar System in both mass and diameter. It’s 18 times less massive than the Earth, and its diameter barely reaches two-fifths of the Earth’s size. To give you a better understanding of Mercury’s size, let’s say that it’s only a third larger than the Moon.
Surprisingly enough, this small planet got even smaller over time. Cooling of its core has caused the entire planet to shrink, reducing Mercury’s volume by about 5-10 kilometers in radius.
How hot is Mercury?
The temperature of Mercury’s surface varies more than on any other planet in our Solar System. It can range from −173 °C (−280 °F) at night to 427 °C (800 °F) during the day. This happens because the smallest planet has almost no atmosphere to trap heat, and it quickly loses the energy received from the Sun during the daytime.
Although Mercury is the closest planet to the Sun, it isn’t the hottest one (Venus is).
Do you know which object is the coldest, the biggest, or the fastest in the Solar System? Which planet hosts the highest mountain, and where is the largest ocean of liquid water (spoiler: not on the Earth)? Take this quiz and learn more about the space record holders!
How long is a day on Mercury?
A solar day — the amount of time it takes the Sun to rise, set, and rise to the same place again — on Mercury lasts about 176 Earth days. And here is another weird fact about Mercury: one year on this planet is twice shorter than a single day and takes 88 Earth days. How come?
The smallest planet spins slowly about its axis — one rotation takes 59 Earth days. However, its sunrises and sunsets are quite different from the Earth’s ones. Due to the planet’s eccentric orbit, the Sun appears to rise twice: once, shortly before setting, and then again from some parts of the surface. The same thing occurs in reverse at sunset. Thus it takes much longer for the Sun to appear in the same place again, and one solar day lasts almost twice as long as a year.
Even though it spins slowly, Mercury travels around the Sun faster than any other planet — at 47 km/s (29 mi/s). In comparison, the second-fastest planet in our Solar System, Venus, has a speed of 35 km/s (21 mi/s).
What is Mercury made of?
Since Mercury is a terrestrial planet, it’s mainly composed of iron, nickel, and silicate rock. It has a large iron core, which is about 61% of the planet’s volume (in comparison, the Earth’s core is only 16%), and a 400-kilometer (250 miles) thick outer shell. By the way, Mercury’s core is about the same size as our Moon. Another similarity with the Moon is that the planet’s dry and rocky surface is full of craters.
Craters are the results of impacts with asteroids or other space objects — the more craters a planet has, the older it is. Since Mercury’s surface is heavily cratered, it’s most likely ancient.
Observing Mercury: a quick guide
Is Mercury visible tonight?
Since the planet is very close to the Sun, there are not many opportunities during a year to observe it. The easiest way to find Mercury in the sky is to use the planetary guide Star Walk 2. Just type the name of a planet in the search field, and the app will show you its position in the sky.
Mercury events 2025
October 29: Mercury at greatest eastern (evening) elongation
On October 29, at 21:59 GMT (5:59 p.m. EDT), Mercury (mag −0.1) will appear farthest from the Sun in the sky: the apparent distance between the two objects will be 23° 54′. The event is called greatest elongation; it is the best time to observe Mercury, as most of the time, Mercury is near the Sun and, therefore, hard to see. Find Mercury shortly after sunset in the constellation Libra.
November 9: Mercury near Antares
- Close approach time: 04:35 GMT (November 8, 23:35 ET)
- Close approach distance: 3°54'
On November 9, Mercury (mag 0.3) will pass close to Antares (mag 1.0) in the sky. Look for them in the constellation Scorpius, shortly after sunset. Viewing conditions will be better in the Southern Hemisphere because the objects will rise higher above the horizon. The Red Planet, Mars, will also be nearby — don’t confuse the twinkling, reddish Antares with the planet.
Mercury and Antares won’t reach conjunction (which means having the same right ascension) because on that day Mercury changes direction and begins retrograde motion.
November 9: Mercury retrograde starts
On November 9, Mercury will enter retrograde motion, meaning that it will appear to move “backward” in the sky until November 29. Read our dedicated article to discover the science behind the retrograde motion phenomenon and whether it’s possible to see it in the sky.
November 12: Mercury near Mars
- Conjunction time: 18:41 GMT (13:41 ET)
- Conjunction distance: 1°18'
- Close approach time: November 13, 03:44 GMT (November 12, 22:44 ET)
- Close approach distance: 1°13'
On November 12, Mars (mag 1.5) and Mercury (mag 1.2) will shine together in the constellation Scorpius, low in the western sky right after sunset. Both planets will be bright enough to spot without optical aid, but because they’ll sit close to the horizon, you’ll need an unobstructed view. Adding to the scene, the red supergiant star Antares (mag 1.1) will glow nearby, creating a beautiful celestial trio.
November 20: Moon near Mercury
- Close approach time: 06:46 GMT (01:46 ET)
- Close approach distance: 5°17'
- Conjunction time: 09:29 GMT (04:29 ET)
- Conjunction distance: 5°29'
On November 20, the Moon in its new phase (0%-illuminated) will pass close to Mercury (mag 6.7) in the sky. Both objects will be located in the constellation Libra. This event will be impossible to observe because both objects will be positioned too close to the Sun in the sky.
November 20: Mercury at inferior conjunction with the Sun
On November 20, 2025, at 08:37 GMT (03:37 ET), Mercury (mag 6.7) will be directly between Earth and the Sun — in astronomy, it’s called an inferior conjunction. At its closest point, Mercury will appear only 0°32' from the Sun, so it will be completely lost in the Sun’s glare for several weeks. This phenomenon occurs approximately every 116 days and marks the end of Mercury’s visibility in the evening sky. Once the planet moves farther from the Sun, it will reappear as a morning object.
November 25: Mercury near Venus
- Close approach time: 00:34 GMT (November 24, 19:34 ET)
- Close approach distance: 0°58'
- Conjunction time: 04:51 GMT (November 24, 23:51 ET)
- Conjunction distance: 1°40'
On November 25, Mercury (mag 2.1) will pass close to Venus (mag -3.9) in the sky. Both objects will be in the constellation Libra, very close to the Sun. Faint Mercury will be invisible due to the Sun’s glare, but bright Venus may be visible very low in the eastern sky just before sunrise.
Remember: Never point your telescope, binoculars, or camera at objects near the Sun — this can cause temporary or even permanent eye damage.
November 29: Mercury retrograde ends
Mercury, which has been retrograde since November 9, will return to its normal, or prograde, motion on November 29. The next retrograde period for Mercury will last from February 25, 2026, to March 19, 2026. See our dedicated article to learn what planets are retrograde right now, and when Mercury will go “backwards” next time.
December 7: Mercury at greatest western elongation
On December 7, at 20:59 GMT (15:59 ET), Mercury (mag -0.5) will be at 20° 42′ west of the Sun in the sky. The planet will be visible in the constellation Libra before sunrise.
December 18: Moon near Mercury
- Conjunction time: 13:51 GMT (08:51 ET)
- Conjunction distance: 6°41'
- Close approach time: 14:25 GMT (09:25 ET)
- Close approach distance: 6°39'
On December 18, the Moon in its new phase (3.1%-illuminated) will pass close to Mercury (mag -0.5) in the sky. The Moon will be in the constellation Scorpius, and Mercury will be in the constellation Ophiuchus. Look for them in the morning, low above the southeastern horizon.
F.A.Q.
What color is Mercury?
Mercury is primarily dark gray. A planet’s color depends on the color of its surface and its reflective abilities. Unfortunately, Mercury’s rocky exterior covered with dust doesn’t give a wide variety of colors.
How many moons does Mercury have?
None. Without going into details, Mercury just cannot form, capture, or acquire a moon due to the planet’s small size, weak gravity, and proximity to the Sun.
How far is Mercury from the Sun?
Its average distance from the Sun is about 58 million km (36 million miles). Thus, among other planets in the Solar System, Mercury is the closest one to the star.
When was Mercury discovered?
We don’t know for sure — the first recorded observation was made in 265 BC. Because Mercury is so close to the Sun, it’s often hidden by its glare. The first people who observed Mercury through a telescope were Galileo Galilei and Thomas Harriot in the 17th century.
Why is Venus hotter than Mercury?
Despite Mercury being the closest planet to the Sun, Venus is hotter because of the thick Venusian atmosphere trapping heat. Mercury, in its turn, has no significant atmosphere and can’t hold the Sun’s energy.
What does the Sun look like from Mercury?
If you could stand on Mercury, at aphelion (the farthest Mercury can get from the Sun), you would see the Sun twice bigger than it appears from the Earth. At perihelion (the closest distance), the Sun would appear three times larger than it does on our planet.
Planet Mercury: to sum up
Mercury is, without a doubt, one of the most extreme planets of all: this planet is the smallest and the closest to the Sun, and the difference between night and day temperatures here makes up about 610°C (1,100°F). Mercury has no moons and almost no atmosphere. This planet is also one of the hardest to observe as it’s near the Sun in the sky most of the time. Share this article with your friends and keep learning more about astronomy with Star Walk 2 and Sky Tonight. You can also watch the fun and educational cartoon about Mercury that explains the main facts about the planet in simple words.
Wishing you clear skies and happy observations!
