Mercury

Mercury is the hardest planet to observe, not because it’s faint, but because it’s never far from the Sun. As the innermost planet, Mercury orbits close to the Sun and never strays more than about 28° away from it in our sky. This means you can only see it low on the horizon just after sunset or just before sunrise - and only for brief windows a few times a year when it reaches maximum elongation.

Most of the time, Mercury is lost in the Sun’s glare or hidden below the horizon. Even when it’s theoretically visible, haze, clouds, and atmospheric turbulence near the horizon make it difficult to spot.

What You Can See

With the naked eye, Mercury looks like a bright star - magnitude 0 to -1 (rarely -2) depending on its distance from Earth. It doesn’t twinkle like stars do because it’s a disk, not a point of light, though you won’t notice that without optical aid. Through binoculars or a small telescope, Mercury shows phases like the Moon and Venus. When it’s near maximum elongation moving away from the Sun, it appears as a fat crescent or gibbous shape. When it’s heading back toward the Sun, it’s a thin crescent.

That’s about it. Mercury is small - only 4,880 km in diameter, about a third the size of Earth - and even at its closest, it’s too far away and too small to show surface features in amateur telescopes. You won’t see craters or markings. Just a tiny, bright, phase-showing disk.

When and How to Observe

Mercury reaches maximum elongation about three times a year in the evening sky and three times in the morning sky. These are your only realistic observing windows. Check an astronomy app or magazine for dates. When Mercury reaches maximum elongation, look for it low on the horizon about 30-45 minutes after sunset (evening elongation) or 30-45 minutes before sunrise (morning elongation). You need a clear, unobstructed horizon - no trees, buildings, or hills blocking the view. Binoculars help enormously. Mercury is bright but small, and picking it out from the horizon glow without optical aid can be tricky. Once you spot it with binoculars, you can aim a telescope if you want to see its phase.

The Bottom Line

Mercury is more of a challenge than a rewarding target. You’re mostly observing it to say you did. It’s the hardest planet to catch, so there’s satisfaction in successfully spotting it, but don’t expect much detail. If you’ve never seen Mercury, make the effort during a favorable elongation. But if you miss it, you’re not missing much. It’ll come around again in a few months, and it’ll look pretty much the same.