Which planet is closest to the sun?

The question which planet is closest to the sun seems like a simple geography question. Yet there turns out to be a surprising amount behind it. For although Mercury is the planet closest to the sun, it is NOT the hottest planet. Moreover, Mercury is hard to see from Earth. In this blog you can read how that is, and how you can still observe it.

Which planet is closest to the sun?

The planet closest to the sun is Mercury. With a distance of only 58 million kilometers from the sun, it is the innermost planet of our solar system. Yet Mercury is far from being the best-known planet - and there's a reason for that. It is small, hard to see, and often skipped by beginners. In this blog, you will read why Mercury is so special, how to find it and why it is NOT the hottest planet.

Mercury: the inner planet

Mercury turns in only 88 days around the sun and barely has an atmosphere. As a result, temperatures fluctuate extremely: from -180°C overnight to +430°C daytime. Because there is no layer of air to retain heat, the night side cools down at lightning speed once the sun goes down.

Why is Venus hotter than Mercury?

You would think that Mercury is the hottest planet - after all, it is closest to the sun. Yet that's not true. Venus is on average warmer than Mercury, with constant temperatures around the 465°C. This is because of the powerful greenhouse effect of its dense carbon dioxide atmosphere. This effectively retains heat both day and night.

Mercury completely lacks such an atmosphere, so heat dissipates quickly. A fascinating contrast between distance and temperature.

Why do you rarely see Mercury?

Mercury is difficult to spot because it is always close to the sun in the sky. As a result, it is only visible:

• Short after sunset (evening elongation)
• Or flat before dawn (morning light)

Even then, you have to give him low on the horizon search, with a completely unobstructed view to the west or east. No trees, buildings or fog - making it challenging for novice stargazers.

Best times to see Mercury

Two to three times a year, Mercury reaches its greatest angular distance from the Sun: the elongation. These are the best times to observe it. The exact dates change every year. You can find them in:

• Our monthly stargazing guide
• Online celestial calendars such as Heavens-Above or Stellarium

Tip: Choose a clear evening or morning, and be on time - Mercury quickly disappears in the glow of the sun.

How do you view Mercury with a telescope?

Mercury can be seen as a bright dot with the naked eye, but you can see much more with a telescope - especially if you know when to look.

Observation tips:

• Use a telescope with minimum 60x magnification
• Choose a good evening elongation
• Find a place with unobstructed view of the western sky
• Use a moon filter to improve the contrast
• Note the phases of Mercury (as with the moon)

These phases occur because we view Mercury from a sharp angle as it orbits the sun. It is one of the few planets where this is easily visible visually.

🔗 Also check out our guide: Looking at planets with a telescope

Mercury in the sky: fast and changeable

Mercury moves one degree per day on average in the sky. As a result, its position changes rapidly. Visible at sunset on Monday, by Friday it may have disappeared.

This speed also explains his name: in Roman mythology, Mercury was the god of speed and communication - a fitting name for a planet that moves so quickly through the heavens.

Did you already know this about Mercury?

• Mercury has no moons
• The surface is full of craters, gorges and valleys
• The planet has the largest temperature differences of all eight
• He is similar to the Moon, but has an iron core
• Mercury is the subject of space research (including by ESA's BepiColombo mission)

Conclusion: a planet for the advanced

Mercury is the planet closest to the sun, but its location and weak visibility make it a challenging object. Still, it pays to seek it out: once you see it, it is an impressive achievement.

Want to observe Mercury yourself? Then choose a telescope with good optics and sharpness at low magnifications. Even models such as the Omegon 102/640 Whether the Celestron StarSense Explorer are suitable for this purpose.