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 it 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. Few key dates to remember while planning your viewing sessions are:
- 23 June 2019 – after sunset
- 09 August – before sunrise
- 20 October – after sunset
- 28 November – 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 first step to planning your observing session is to know if and when Mercury plans to visit our night sky. This guide lists out where the planets are this month (link opens in a new tab).
In April, August and November – Mercury will be visible in the Eastern sky, rising an hour or so before sunrise. Local latitude, season, and the planet’s current apparent magnitude will play a major role in deciding its visibility.
You can check here if Mercury is visible from your specific location by using astronomy software. Free software like Stellarium will do the job just fine. We recommend Sky Safari, which is paid but gives more detail and control.
On 9th August 2019, greatest elongation will only have Mercury 8° above the horizon 45 minutes before sunrise, making this one a tricky spot.
After August, it will again reach maximum western elongation on 28 November 2019. This offers better seeing with the planet rising to around 10° above the eastern horizon 45 minutes before sunrise.
February, June, and October – Mercury will be seen sinking towards the Western horizon an hour or so after sunset.
It reaches greatest eastern elongation on 23 June 2019, and can be found 9° above the horizon 45 minutes after sunset from latitudes around 45°N. It is also satisfyingly close to a faint mars on this evening too!
On 20 October 2019, Mercury again reaches greatest eastern elongation, but this is a poor one for mid-latitude observers. Mercury will practically be setting 45 minutes after the sun, making it effectively unobservable.
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.
Below is the list of constellations where Mercury can be found during its greatest elongations. Note that while constellations themselves will be hard to see (if not impossible) because of twilight, you can use them as a guide to know which part of the sky to look at.
Morning Apparition (Western Elongation)
28 November 2019 – 20° from the sun, look towards Libra. Best viewed from Northern Hemisphere.
Evening Apparition (Eastern Elongation)
23 June 2019 – 25° from the sun, look towards Gemini. Best viewed from Southern Hemisphere.
20 October 2019 – 24° from the sun, look towards Libra. Best viewed from Southern Hemisphere.
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.
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.
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 directly looking at the sun when hunting for Mercury.
Not only will direct sunlight can also ruin your telescope, more importantly, seeing it through a telescope will permanently blind you!
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.
Mercury’s Solar Transit in 2019
A rare exception to the conventional conjunctions occurs on 11 November 2019 when Mercury’s orbit takes it close to inferior conjunction, producing a transit across the face of the Sun.
During a transit and with the right equipment, we can see the little Mercurian disc cross the face of the daytime sun. The video below shows the last one, from 2016 in action.
The 2019 transit will commence at 12:35 UT and end at 18:04 UT on the same day. It will be visible from anywhere in the world, as long as the Sun is above the horizon.
It’s an event well worth trying to see as these transits only occur 13 times in a century and the next one is not until 2032.
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