Have you ever seen the Solar System’s smallest planet with your own eyes?
An inner planet that pings around the Sun in a mere 88 days, the little planet Mercury is a tricky one to see from Earth.
Why? Since it orbits around the Sun very closely, Mercury is almost permanently in our daytime sky, but almost always lost in its glare.
Only when it appears at its furthest point east and west of the Sun can it be visible either just before sunrise or just after sunset.
The latter happens this coming week as Mercury reaches what astronomers call its “greatest eastern elongation.”
Here’s everything you need to know to find the Solar System’s smallest planet with your own eyes.
When to look for Mercury
The key evenings are from this Saturday, January 23, 2021—as Mercury reaches its greatest eastern elongation—through Tuesday, January 26, 2021 when the little planet will appear at its highest above the western horizon at roughly 15º, depending on your location. The few nights after that will also be good nights to look.
It will become visible in a clear sky from about 30 minutes after sunset where you are, and it will set about an hour later—so you’re going to have to be quick to catch the “Swift Planet.”
The further north you are the longer your window of opportunity because of the differing angle of inclination of the ecliptic to the horizon. The further north you are, the narrower that angle, so the higher-up Mercury appears.
What is an ‘elongation?’
It’s the “angular separation”—the gap between—a planet and the Sun as seen from Earth. Only when it’s near the maximum length is it possible for Mercury to be seen easily.
It’s just a line-of-sight thing since Mercury is just doing what it always does in orbiting the Sun every 88 days.
There are two kinds of elongations—western and eastern—which alternate every couple of months. The only difference between these two farthest apparent distances from the Sun is that they make Mercury visible at different times of the day from Earth.
Mercury’s greatest eastern elongation is visible to us on Earth in the west after sunset (its so-called “evening apparition”), while its greatest western elongation is visible to us in the east before sunrise (its “morning apparition”). Confusing, right?
This weekend’s greatest eastern elongation will see Mercury 18.6º east of the Sun in the west after sunset.
How to find Mercury in the night sky
You need to observe from somewhere with a reasonably low view of the southwestern horizon. Get in position just after sunset and, as twilight sets in, Mercury will suddenly become visible as a small dot of light about 15º above the horizon. Be patient!
Binoculars will help, but don’t even think of using them before the Sun has completely set.
What else to look at in the night sky this weekend
Aside from Mercury there will be a couple of other objects you can easily see in the night sky this weekend.
Look to the southeast anytime after dark and you’ll see a a waxing gibbous Moon while if you glance high in the southeast you’ll also find Mars.
When next to see Mercury in 2021
There are five other Mercury-viewing opportunities not to miss in 2021:
1. Jupiter and Mercury in a close conjunction
When: Just before sunrise on Friday, March 5, 2021
Where: close to the east-southeastern horizon
The two planets will appear to be a mere 0.3° apart just before sunrise, which promises to be a lovely sight. Dimmer Saturn will be just above the and to the right of the planetary pair.
2. A planetary trio and the Moon
When: Just before sunrise on Tuesday, March 9 and Wednesday, March 10, 2021
Where: close to the southeastern horizon
On successive mornings you’ll be able to see Mercury align with Jupiter and Saturn, with a slim crescent Moon in attendance for good measure. Beautiful!
3. Mercury’s next eastern elongation
When: Just after sunset from Saturday, May 13 through Monday, May 29, 2021
Where: close to the western horizon
Mercury’s next “greatest eastern elongation” is another good opportunity to see Mercury with your own eyes. During this time Mercury will rise high in the western sky and reach about 15º above the horizon after sunset.
4. Venus and Mercury in a close conjunction
When: Just after sunset on Monday, May 29, 2021
Where: close to the western horizon
Although the two inner planets can be tricky to observe from Earth, they pass each other frequently. Today they will be appear to be just 0.4º apart while 17º from the Sun.
5. Mercury may be visible during a total solar eclipse
When: Just after sunrise on Saturday, December 4, 2021
On December 4, 2021 there will be a total solar eclipse in Antarctica. It won’t be witnessed by many—just those on cruises around Antarctica and a lucky few eclipse-chasers in planes—but those viewing totality may be able to see Mercury close to the Sun.
Times and dates given apply to mid-northern latitudes. For the most accurate location-specific information consult online planetariums like Stellarium and The Sky Live. Check planet-rise/planet-set, sunrise/sunset and moonrise/moonset times for where you are.
Wishing you clear skies and wide eyes.