Seeing Mercury with a telescope is a big challenge!
While it occupies a place in the list of planets that can be seen with unaided eyes, spotting it can get quite difficult because Mercury is so close to the sun. The positive side of its solar proximity is its fast orbit.
Mercury races around the sun every 88 days, which means we get several short viewing windows every year.
The downside of being so close to the sun is that it is never visible in a truly dark sky – only dawn and dusk – and then only for a few days at a time. Most of the year it is too close to the sun for us to view it.
However, Mercury can be viewed if you know where and when to look. While backyard telescopes don’t have enough resolution to fetch details like impact craters and lava fields, there is still so much to look at and enjoy!
When Is Mercury Visible?
It is often said to be elusive, but not because it is dim. In fact, at its brightest, Mercury shines at a magnitude of -1.9, which is brighter than Sirius, the brightest star in our night sky!
Mercury is illusive because it is only visible for a few days each time it reaches maximum separation from Sun, moments referred to as greatest elongation. Due to its tight orbit, Mercury can only go as far as 28 degrees from the sun even at its maximum elongation.
To put that into perspective, hold your arm out straight in front of you and open your fist such that your little finger and thumb are stretched as much as possible with the other fingers curled down. The tip of your little finger and your thumb will span about 25 degrees. (Learn more about using your hands to measure angles.)
That’s about as far away from the sun as Mercury ever ventures! And this tiny world needs to be a minimum of 12° away from our local star to be visible against the sunlight. All of which means that twilight is the only time when it can be viewed, either an hour before sunrise or after sunset.
This year, Mercury reaches maximum elongation 6 times, alternating between morning and evening skies depending on whether it lies to the east of the Sun or to the west.
The dates for its greatest elongation on 2021 are:
- 23 January – after sunset
- 06 March – before sunrise
- 17 May – after sunset
- 04 July – before sunrise
- 13 September – after sunset
- 25 October – before sunrise
Now you know when the best times are for seeing Mercury, let’s look at the best way of doing it.
How to find Mercury in the Night Sky
Step 1 – Is Mercury in the Night Sky Tonight?
The brief guide to its greatest elongations shows the best times to look for Mercury in 2021. Visibility is usually fair to good for a week or so either side of the greatest elongation date.
For viewing in the morning, you need to be facing the eastern horizon with a clear view.
Try and see the planet an hour or so before sunrise. Although Mercury can shine brightly, the glow of dawn quickly overwhelms it.
The angle of the ecliptic has a huge influence on how high above the horizon little Mercury rises. For instance, it is only 4° above the horizon 40 minutes before sunrise at March’s greatest elongation, 6° at June’s, but a decent 10° high at the end of October.
Unsurprisingly, see Mercury in the evening means facing the western horizon so we can be prepared for it to come into view as the Sun sinks lower under the horizon.
Look for it at the end of January after sunset. The best views of the year are presented in the middle of May. At that point, Mercury will be a full 12° above the horizon 40 minutes after sunset, affording great viewing with a planetary telescope.
The last evening views of 2021 are in September, but the planet is very low on the horizon after sunset and not great for observing.
Step 2 – Finding Mercury Without a Telescope
In spite of being one of the 5 planets that can be without an optical aid, Mercury is far more difficult to spot than other bright planets because of its proximity to the Sun.
During its better apparitions, those when it is highest in the sky 45 minutes after sunset or before sunrise, it can be seen with the naked eye or simple binoculars.
Look towards the general direction of sunset in the evening, i.e. westerly or sunrise in the morning, i.e. easterly.
Although Mercury is generally white in color, its proximity to the horizon gives off a pinkish hue. This follows the same logic as to why our Sun appears reddish during sunrise and sunset.
While you can locate Mercury with the naked eye, no detail is visible.
NEVER attempt to hunt for Mercury whilst the sun is still above the horizon or immediately before sunrise. It is too easy to accidentally end up looking directly looking at the Sun which can permanently blind you!
Step 3 – Finding Mercury With a Small Telescope
Once you locate Mercury with your eye, slew your telescope so that it points in that general direction.
Your finderscope has a wider viewing range, so use that to get it centered in your main scope. When you look through the finder, you should be able to see crosshairs, which have to be centered at Mercury.
Now, look through the eyepiece of your telescope to find a magnified view.
Targets to Look For When Observing Mercury with a Small Telescope
Like our moon, Mercury presents us with phases, which can only be viewed when Mercury is not in either inferior or superior conjunctions.
Inferior conjunction occurs when Earth, Mercury, and Sun are aligned, i.e., the planet is between Earth and Sun. Superior conjunction occurs when it is on the opposite side of the sun.
During these conjunctions, Mercury is completely submerged in the sun’s glare and cannot be viewed. Instead, the first and last quarter of its phases can be observed at greatest eastern and western elongations respectively. That’s good news for us, because they are more interesting to see!
To improve viewing, consider using color filters with Wratten Numbers similar to #21 Orange, which help in reducing sky’s brightness.
For more information on how much of its surface is illuminated during its phases, we recommend Sky & Telescope magazine, or the software we linked to earlier in this article.
It might be the most elusive planet in our solar system, but it is also one of a kind.
While observing Mercury might not be as easy as spotting Jupiter or Saturn, Mercury’s phases are worth checking out, not to mention the prestige and sense of accomplishment that comes from seeing a rarely glimpsed planet.
Written by Sharmila Kuthunur