Finding Mercury

Finding the planets in the night sky can be a fun task. Even with the naked eye, five of the planets can be spotted with relative ease. However, the trickiest of the naked eye planets to spot is Mercury, the closest world to the Sun.

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Mercury as viewed through a telescope. Image taken by me

Whereas Venus and Jupiter are unmistakably bright and Mars has a red hue, Mercury isn’t always so easy to find. First, since Mercury is so close to the Sun, it can be lost in twilight some of the time. Also, since Mercury is inside of Earth’s orbit, Mercury appears in crescent-like phases like our Moon and Venus. Since Venus has a thick atmosphere, it is able to reflect a great deal of sunlight and appear bright. Mercury has no real atmosphere to speak of, and so the sunlight reflects off of rock, thereby appearing more dim.

Mercury is easier to spot during certain times of year. When Mercury reaches the farthest point from the Sun in its orbit (its greatest elongation) close to the Spring equinox, it is favorable to see in the evening sky. Likewise, when Mercury reaches its greatest elongation near the Fall equinox, Mercury is favorable to spot in the morning. This is because of the tilt of the Earth’s axis.

The Sun, Moon, and the planets travel across the sky in a plane called the ecliptic. Near the Spring equinox, the ecliptic intersects the western horizon at a steeper angle, which allows us to view Mercury in a more perpendicular angle relative to the Sun. This allows Mercury to appear above the Sun, and Mercury appears higher in the sky at sunset.

Conversely, during the Fall equinox, the ecliptic intersects the eastern horizon at a greater angle, allowing us to see Mercury above the Sun at dawn. At other points throughout the year, when the ecliptic intersects the horizon at a more shallow angle, Mercury becomes more difficult to find as it appears more parallel to the horizon, and can be lost in the Sun’s glare.

Mercury in crescent phase

Mercury will reach its greatest elongation on April 18, which gives us a great opportunity to spot it in the western sky after the Sun sets. Mercury will be the first star that can be seen in the western sky after the Sun sinks below the horizon, and definitely complements the beauty of the sunset! While not extremely bright, it is still bright enough to be easily spotted. It will have an orange-like color to it, and won’t be very high, anywhere from 15-20° above the horizon. If you hold your fist at arm’s length with your pinky finger towards the ground, that is roughly 10°.

If you are viewing Mercury with binoculars or a telescope, it is recommended that you wait until the Sun is below the horizon so you don’t inadvertently look at the Sun and go blind. Mercury will appear as a crescent, as Mercury is on the opposite side of the Sun from Earth when it appears full. Not a great amount of detail can be seen, as Mercury is low on the horizon and extremely prone to atmospheric disturbances. Still, it’s pretty amazing to see Mercury in a crescent phase, and it is also proof that Mercury is closer to the Sun than Earth.

Caloris Basin impact crater

Mercury has the fastest orbit of all the planets, making one trip around the Sun every 88 days. It passes from an evening star to a morning star every couple of months. The temperature changes from the day side to the night side on Mercury are amongst the most extreme in the solar system, ranging from -280° F on the night side to 800° F on the day side. For every two years that pass on Mercury, only 3 days pass due to an orbital resonance with the Sun. Mercury is also quite small, being only slightly larger than the Moon. In fact, Jupiter’s moon Ganymede and Saturn’s moon Titan are larger in size than Mercury, although not at massive (dense).

Mercury also contains one of the largest impact craters in the solar system- the Caloris Basin. The Caloris Basin is approximately 960 miles across, and it’s surrounded by mountains 1.2 miles high. The impact probably occurred during the Late Heavy Bombardment period around 3.8 or 3.9 billion years ago. It is thought that the impacting object was at least 60 miles in diameter, and the seismic waves converged on the opposite side of the planet, causing a “wrinkly” terrain of sorts.

False color NASA image of Mercury showing different terrain features

Compared to the other naked eye planets, Mercury can be the most tricky to spot. But finding Mercury in the night sky is well worth the effort, as you will have bragging rights and you will have spotted the closest planet to the Sun. Since Mercury zooms around the Sun so fast, Mercury is also frequently in conjunction with other planets as well, giving you the opportunity to see multiple planets in the same field of view. So, if you’re out watching the sunset over the next few weeks, see if you can find Mercury in the west!

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