The Ultimate Guide to Observing Mercury
July 22, 2020
When we talk about our favorite planets to observe through telescopes, which ones most often come to mind? Of course, there’s Saturn and its magnificent rings, and Jupiter with its distinct cloud bands, Great Red Spot, and its four brightest moons that can be seen racing around the planet. Then there’s Mars, named after the god of war with its reddish appearance, unique surface markings, and polar ice caps. And finally, there is Venus, Earth’s twin (size wise), which resembles a mini version of our own Moon as it goes through its phases.
But we rarely hear about the planet closest to the Sun—the smallest and most elusive planet in our Solar System—Mercury. Mercury may be one of the least observed planets, but it can be safely observed during favorable periods when it reaches its greatest separation from the Sun. Astronomers refer to this as “greatest elongation.” Because Mercury is so close to the Sun, elongations occur every few months, placing the planet on either the east or west side of the Sun.
So how do we know when and where to look to find Mercury? Is it dangerous to observe this tiny planet, positioned so close to the Sun? What will I see when I view Mercury through a telescope? What should I look for? Here are a few observing tips to help you get better acquainted with our mysterious, rarely seen neighbor.
Tip #1: ONLY attempt to view Mercury during a safe viewing opportunity
Because Mercury never strays too far from the Sun, it is often lost in the Sun’s glare. Never, ever attempt to locate and observe Mercury in broad daylight as it is extremely dangerous and not worth the risk of accidentally blinding yourself and causing permanent eye damage. Only during a favorable separation from the Sun is it relatively safe enough to observe Mercury in a deep twilight sky when the Sun is below the horizon.
Mercury will be visible in the evening and morning skies during these periods in 2021:
Evening: January 10 to January 31
On January 10, Mercury, Jupiter, and Saturn will come close together for a rare triple conjunction event and form a small triangle low in the southwestern sky. Look 30 to 45 minutes after sunset. Binoculars will help.
On January 24, Mercury reaches greatest eastern elongation of 18.6 degrees from the Sun. This is an ideal time to view Mercury since it will be at its highest point above the horizon in the evening sky.
Morning: from February 28 to March 20
On March 6, the planet Mercury reaches greatest western elongation of 27.3 degrees from the Sun! This is a great time to view Mercury since it will be at its highest point above the horizon in the morning sky. Look for Mercury low in the eastern sky just before sunrise. Note: On the morning of March 5 prior to sunrise, Mercury will appear just to the left of Jupiter. Binoculars will help.
Evening: from May 3 to May 24
On May 17, Mercury reaches greatest eastern elongation of 22 degrees from the Sun. This is an ideal time to view Mercury since it will be at its highest point above the horizon in the evening sky. Look for Mercury low in the western sky just after sunset.
Morning: from June 27 to July 16
On July 4, Mercury reaches greatest western elongation of 21.6 degrees from the Sun. This is an ideal time to view Mercury since it will be at its highest point above the horizon in the morning sky. Look for Mercury low in the eastern sky just before sunrise.
Evening: from August 31 to September 21
On September 14, Mercury reaches greatest eastern elongation of 26.8 degrees from the Sun! This is a great time to view Mercury since it will be at its highest point above the horizon in the evening sky. Look for Mercury low in the western sky just after sunset.
Morning: from October 18 to November 1
On October 25, Mercury reaches greatest western elongation of 18.4 degrees from the Sun. This is an ideal time to view Mercury since it will be at its highest point above the horizon in the morning sky. Look for Mercury low in the eastern sky just before sunrise.
So now that you know when Mercury will be visible, you’ll need to determine where it is in the sky.
Tip #2: Use an astronomy app or star chart
Star charts in astronomy-related magazines, books, or websites are visual guides to help you plan your quest to spot Mercury. The most modern and informative tools today can be found in astronomy apps such as Celestron’s SkyPortal mobile app. Simply download this free planetarium app from the Apple App Store or Google Play, and you’ll instantly have a wealth of information at your fingertips. Not only does SkyPortal provide audio and written descriptions about Mercury, it also provides its celestial coordinates, a real-time sky map, rise and set times, physical and orbital parameters.
Tip #3: The best equipment for viewing Mercury
While it is rewarding to spot Mercury with your naked eyes, a pair of low powered binoculars can help you locate it as soon as the Sun dips safely below the horizon and the sky begins to fade into twilight. Mercury will be low on the horizon, in a part of the atmosphere that is thicker and more susceptible to atmospheric turbulence. That means Mercury will look like a shimmering or twinkling “star” as it shines through the unstable air.
If possible, use a telescope of at least 2.4” to 4” of aperture, especially one with GoTo and tracking, which will keep Mercury centered for a more stabilized view.
Tip #4: What to look for while observing Mercury
Because Mercury is an inferior planet whose orbit is closer to the Sun than the Earth’s, it displays a full range of phases—viewable only through a telescope—just like a smaller version of our Moon. You’ll be able to view Mercury's phases in a small telescope with moderate magnification if seeing conditions are ideal.
Mercury has a reputation for being a challenging telescope target, so it may take a few tries before you’re able to observe it. But your patience will pay off; it is the only terrestrial planet other than Mars that reveals some detail on its surface through larger aperture telescopes. However, do not expect to resolve any craters because Mercury is just too small.
We hope this brief Mercury viewing guide will be of some help on your mission to observe this elusive little planet in our Solar System. The key is knowing when and where to look. If you wish to learn more about Mercury, please download the SkyPortal mobile app from the App Store or Google Play. Clear skies!