The sky — astronomy

Mercury: The Planet Closest to the Sun

Mercury is the smallest planet in the Solar System and the closest to the Sun, completing an orbit in just 88 Earth days. Its surface swings from about 430 °C by day to about −180 °C at night. It is hard to spot because it never strays far from the Sun, so look low in the sky shortly after sunset or before sunrise.

Mercury at a Glance

Mercury is the innermost and smallest of the eight planets. Its diameter is about 4,879 km, roughly 40% larger than that of our Moon and a little over a third of Earth's. Surface gravity is about 38% of what we feel on Earth.

Its average distance from the Sun is about 58 million km, but the orbit is noticeably stretched. At its closest, Mercury is around 46 million km from the Sun; at its farthest, around 70 million km. One lap takes 88 Earth days, which makes it the fastest planet. The Romans named it after the swift messenger of the gods.

Mercury has no moons and no rings. It is very dense, and studies indicate that a large metallic core makes up much of its interior.

Surface and Temperatures

Mercury is covered in craters and looks a lot like the Moon. One of its largest features is the Caloris Basin, about 1,500 km across, left by a huge impact long ago. Long cliffs also run across the surface, which scientists link to the planet cooling and shrinking over time.

The planet has almost no atmosphere, only an extremely thin exosphere (a tenuous outer envelope of scattered atoms). It cannot hold heat, so temperatures are extreme: around 430 °C on the sunlit side and down to about −180 °C at night.

Surprisingly for a world this close to the Sun, Mercury has water ice. It sits in deep polar craters whose floors never see sunlight. Observations, including those from NASA's MESSENGER mission, support this.

Mercury also spins slowly. One rotation relative to the stars takes about 59 Earth days, but because of its fast orbit, one solar day (sunrise to sunrise) lasts about 176 Earth days. The planet turns exactly three times for every two orbits.

Why Mercury Is Hard to See

Mercury is bright, yet elusive. Because its orbit lies inside Earth's, it always appears close to the Sun in our sky. You will never see it in the middle of the night. Instead, look low in the west after sunset or low in the east before sunrise.

Some practical tips:

• Choose a spot with a clear, flat horizon, free of buildings and trees.
• Start looking roughly 30–60 minutes after sunset (or the same time before sunrise), when the sky is dark enough but the planet has not yet dropped below the horizon.
• Mercury looks like a bright, pale, steady point that does not twinkle much. Never point binoculars toward the Sun; wait until it is fully below the horizon.
• Mercury moves quickly from night to night, so it is worth trying several evenings in a row.

Like the Moon, Mercury shows phases, but you need a telescope to see them. For general observing advice, see how to watch the sky.

Greatest Elongation

Elongation is the angle between a planet and the Sun as seen from Earth. Greatest elongation is the moment Mercury appears farthest from the Sun, and so the best opportunity to see it.

The angle is not always the same because Mercury's orbit is stretched: it ranges from roughly 18° to 28°. A larger angle generally means Mercury stays in a dark sky longer after sunset or before sunrise.

There are two kinds:

• Eastern elongation — Mercury lies east of the Sun and shows up in the evening sky after sunset.
• Western elongation — Mercury lies west of the Sun and shows up in the morning sky before sunrise.

Mercury reaches greatest elongation several times a year. The angle alone does not decide how good the view is. The tilt of the ecliptic (the Sun's apparent path across the sky) against your horizon also matters, because it determines how high the planet climbs in the twilight. A smaller elongation with a steeper ecliptic can beat a larger one with a shallow angle. For exact dates at your location, check the Astronativa 3D sky or a planetarium app.

Missions to Mercury

Mercury is a difficult target. A spacecraft falling toward the Sun gains speed, and slowing down enough to be captured takes a great deal of fuel and clever navigation.

• Mariner 10 (NASA) flew past in 1974–1975 and imaged about half of the surface.
• MESSENGER (NASA) became the first spacecraft to orbit Mercury, in 2011. It mapped the whole surface, studied its composition and magnetic field, and helped confirm polar ice. The mission ended in 2015 with a planned impact on the surface.
• BepiColombo (ESA and JAXA) is a joint European–Japanese mission launched in October 2018. It consists of two orbiters and uses flybys of Earth, Venus and Mercury to slow down. Entry into Mercury orbit is planned for November 2026.

Mercury has a weak but real magnetic field. Among the questions BepiColombo aims to help answer are how that field is generated and what the planet's interior looks like.

From Earth, Mercury sometimes appears to move backward against the stars. This is an optical effect caused by the different orbital speeds of the planets. Read more in retrograde motion and why planets seem to go backward. Cultural interpretations are a separate topic, covered in Mercury in astrology.

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