2021 Mercury Greatest Elongations

MERCURY'S GREATEST ELONGATIONS OF 2021

Wide FOV Image Credits: NASA/JPL Solar System Simulator developed by: David Seal

L O A D I N G

As an Inferior Planet, one whose orbit is smaller than Earth's, Mercury can never move completely around our home planet! This means that Mercury—the fleetest of all the major planets—repeats a cycle through our earthly skies that makes it "appear" to zig-zag back and forth past the Sun! Current Mercury.

The farthest eastern and western angular distances that Mercury achieves from the Sun are called its Greatest Elon- gations, and in a single year generally six or seven Greatest Elongations of Mercury occur. Click on the labels above for various views of Mercury's six Greatest Elongations of 2021. The dates given are the best times to view this elusive planet, which often appears so close to the Sun that it is lost in the brightness of our parent star. You can view Mercury's Greatest Elongations with or without orbits, and you can zoom in on the fleet planet to see "close-up" on those dates. Incidentally, in 2021 none of the examples occur during one of the two times each year (early May and early November) when Mercury's orbit appears "edge on" to us here on Earth. But in the future keep an eye out for those that do. You can find Mercury's Greatest Elongations hundreds of years in the past and future with the Mercury Chaser's Calculator.

Remember that Mercury and Venus are the only two major planets that exhibit greatest eastern and western elon- gations because they are Inferior Planets. In contrast, the Superior Planets, whose orbits are larger than Earth's, can at times appear directly opposite the Sun in our skies. If you are viewing Mercury on the date of a Greatest Elongation, also check out its Current Location in the Constellations and Current Location in Orbit, and compare these to what you see in the above views.

WARNING ! Mercury always appears close to the Sun. As It is never safe to look directly at the real Sun with the naked eye, and as looking at the Sun through a telescope or binoculars without adequate safeguards—even for an instant—can cause permanent blindness, NEVER attempt to view Mercury while the Sun is visible! With a telescope or binoculars especially, you run the risk of inadvertantly focusing on the Sun! To learn how to safely "observe" Mercury, consult the professionals at your local planetarium or observatory.

When you examine the above views—all shown at the same scale and with the same FOV (Field of View)—it is easy to conclude that Mercury's orbit is considerably elliptical. As the year progresses and Earth moves in its orbit around the Sun, Mercury's successive greatest elongations can be seen to be irregular, both in their timing and in their apparent angular distances from the Sun. This is because from Earth we see Mercury's oblong and tilted orbit at different angles in different months. For example, one after another look at the succeeding "Eastern" Elongations presented above. Their varying angular distances from the Sun are quite obvious.

As varied as these angular distances are, from our earthly perspective Mercury can never appear more than 28° from the Sun! This fact has important implications concerning the times when Mercury can be observed in our skies. Whenever Mercury is east of the Sun it is only visible at or just after sunset, and whenever it is west of
the Sun it is only visible at or just before sunrise. At all other times Mercury is either a.) outshined by the Sun's brilliance and therefore invisible to us, or b.) below our horizon. For these reasons most persons have probably never seen this enigmatic planet! In fact, due to Mercury's cycle—of appearing east of the Sun, then disappearing, then appearing west of the Sun, then disappearing again—the ancients believed that Mercury was two planets!

Now you may think that Mercury is easiest to observe when it "appears" farthest from the Sun. But this is not necessarily so. Due to the varying "apparent" tilt of the Ecliptic as Earth rotates on its axis and orbits the Sun, there are times when a narrower Greatest Elongation presents more favorable viewing opportunities than a wider one! The images
to the left and below show us why this is so.

When Mercury is west of the Sun, it is visible only at or before sunrise—never for more than 2 hours, and most often for less. Moreover, viewed to the east near sunrise in the North- ern Hemisphere's autumn, the Ecliptic is more vertical at the horizon than it is earlier in the year. This is simply a consequence of Earth's 23.44° tilt relative to its own orbit. As the ma- jor planets never wander far from the Ecliptic, viewed to the east near sunrise, Mercury's or- bit too appears more vertical in the Northern Hemisphere's autumn. In these situations, if Mercury is at Greatest "Western" Elongation,

it is higher above the horizon and visible in the sky longer "before the Sun rises". So, in the Northern Hemisphere Mercury's Greatest "Western" Elongations are easier to witness later in the year!

The opposite situation occurs for Mercury's Greatest "Eastern" Elongations. It is in the Northern Hemisphere's spring near sunset, when—looking west—the Ecliptic is more vertical to the horizon. So, if Mercury is east of the Sun, it is higher above the horiizon and visible in the sky longer "after the Sun sets". This means that in the Northern Hemisphere Mercury's Greatest "Eastern" Elongations are easier to witness earlier in the year.

This is half of the story. As the seasons of
the Northern and Southern Hemispheres are opposites, their most favorable times for observing Mercury's Greatest "Eastern" and "Western" Elongations are also opposites, due to their geometry relative to the Ecliptic.

Comparing Mercury's Greatest Elongations of January 24 and March 6 provides a good example of what is noted above for the Northern Hemisphere. On Jan 24, when the fleet planet is nearer perihelion, it appears only 18.6° "east" of the Sun. Yet an observer at 40° North Latitude is able to view Mercury for about an hour and a half "after the Sun has set". However, on Mar 6, when Mercury is nearer aphelion, it appears a whopping 27.3° "west" of the Sun. Yet an observer at 40° North Latitude is able to view it for only about an hour "before the Sun rises!" Of course, for an observer at 40° "South" Latitude, the opposite is true. So it is easy to see that the time of year and your latitude both matter, if you plan to observe Mercury's Greatest Elongations.

Now all this talk of east and west, and Eastern and Western, and where to look in the sky, all might seem a little confusing at first. Just keep in mind that the terms "Eastern" and "Western" refer to where Mercury appears in
our skies "relative to the Sun". To view Mercury's Greatest "Eastern" Elongations, therefore, you look toward the "west" in the early evening, because that is when Mercury is visible in the sky after sunset! Conversely, to view Mercury's Greatest "Western" Elongations, you look toward the "east" in the early morning, because that is when Mercury is visible in the sky before sunrise! The chart below clarifies this, as well as the best times in each hem- isphere to observe both types of Mercury's Greatest Elongations.

MERCURY'S GREATEST ELONGATIONS
are easier to witness . . .
type Mercury is . . . are viewed . . . in N. Hemis in S. Hemis
"Western" west of Sun before sunrise (looking east) later in year earlier in year
"Eastern" east of Sun after sunset (looking west) earlier in year later in year

You can gain further insight about Inner Planet elongations with this Configurations Simulator and this Planetary Phases Simulator. In both simulators' Orbit Sizes boxes, select Earth as "observer" and Mercury as "target".

And while you're at it checking out Mercury's Elongations, don't forget that you can also find when viewing the "other" Inferior Planet is optimum. Check out Venus's Greatest Elongations of 2021.

Simulator Image Notes: 1) Fields of View are measured horizontally. 2) Eclipse shadows of planetary moons are not displayed. 3) Light-Travel Time is not included in calculated times. 4) Planet of focus (at center of simulator image) is viewed with its local north "upward", i.e. in the 12:00 midnight direction. 5) Simulator expresses the phase (where applicable) as the absolute value of the phase angle.

Note: zoomed views of Mercury were generated with Celestia 1.6.1 and show accurate representations of the planet's surface.

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