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Mercury’s greatest morning elongation on March 24

Mercury, the solar system’s innermost planet, reaches its maximum elongation from the sun for 2020 this week. At maximum on March 24, this world resides a whopping 27.8 degrees west of the sun, placing Mercury in the morning sky before sunrise.

You might think right now should present the best time to see Mercury in 2020. Yes, that’s the case if you live in the world’s Southern Hemisphere. From the Southern Hemisphere now, before sunup, you will find a glorious sight awaiting you. Three planets – Jupiter, Mars and Saturn – are clustered high above the sunrise point, while Mercury shines below them, nearer the twilight. Northern Hemisphere? See the chart above.

The planets align with the ecliptic (Earth’s orbital plane projected onto the sky’s dome) in the same way in both the Northern and Southern Hemispheres … except that, for the northern part of Earth, the ecliptic or pathway of the sun, moon and planets (the green line on our charts) makes a narrower angle with respect to the morning horizon. (Contrast the Northern Hemisphere chart above with the Southern Hemisphere chart below.)

Thus, at northerly latitudes, the planets are to the right of the sunrise point on the horizon instead of above it. They’re lower down in the predawn/dawn sky. Saturn, Jupiter and Mars will still be easy to see. But Mercury will be tougher, in deeper twilight. If you live as far north as Alaska, you probably won’t see Mercury at all – even with binoculars – where the sun and Mercury rise at almost the same time.

In short, the farther south you live, the more time before sunrise that Mercury rises. We give the approximate rising times for Mercury at various latitudes (given a clear and obstructed horizon in the direction of sunrise):

40 degrees north latitude: Mercury rises about one hour before the sun
Equator (0 degrees latitude): Mercury rises about 1 2/3 hours before the sun
40 degrees south latitude: Mercury rises about 2 1/4 hours before the sun

Want more specific information? Click here for a sky almanac.

Sucks to be in the Northern Hemisphere? Nah! Consider that – also on March 24, 2020, same date Mercury swings out to its greatest morning elongation – the brightest planet Venus reaches its greatest elongation in the evening sky. It’s the springtime sky for the Northern Hemisphere, and the northerly latitudes enjoy the better view of Venus right now than do the southerly latitudes. Read more: Venus’ greatest evening elongation on March 24

Nearly vertical line of ecliptic in dawn sky with Jupiter, Mars and Saturn at top and Mercury close to the horizon.
On early autumn mornings the ecliptic – the pathway of the planets – hits the horizon at a steep angle. Therefore, in the Southern Hemisphere, where it’s now autumn, the planet Mercury is easier to see than at mid-northern latitudes. The Northern Hemisphere view can be seen on the featured chart at the top of this post.

Mercury presents six greatest elongations in 2020, three in the evening sky and three in the morning sky. Because Mercury’s orbit is quite eccentric (oblong), Mercury’s greatest elongations are far from equal throughout the year, varying from about 18 to 28 degrees. We list all of Mercury’s greatest elongations for 2020:

Mercury greatest elongations in 2020

February 10, 2020: 18.2 degrees east of the sun (evening sky)
March 24, 2020: 27.8 degrees west of the sun (morning sky)
June 4, 2020: 23.6 degrees east of the sun (evening sky)
July 22, 2020: 20.1 degrees west of the sun (morning sky)
October 1, 2020: 25.8 degrees east of the sun (evening sky)
November 10, 2020: 19.1 degrees west of the sun (morning sky)

You might wonder why the extreme difference between the Northern and Southern Hemispheres for viewing Mercury in the morning sky. It’s because it’s now early spring in the Northern Hemisphere, yet early autumn in the Southern Hemisphere. The recent equinox on March 20, 2020, was the Northern Hemisphere’s spring equinox and the Southern Hemisphere’s autumn equinox.

Read more: Does the equinox sun rise due east and set due west?

At sunrise on and around the spring equinox, the ecliptic – the roadway of the planets on the sky’s dome – hits the sunrise horizon at its shallowest angle for the year. Therefore, Mercury – even at its greatest morning elongation – never gets far from the sun’s glare around the spring equinox, so it sits low in the glow of morning twilight.

At sunrise on and around the autumn equinox, the ecliptic – the planetary pathway – hits the sunrise horizon at its steepest angle for the year. Therefore, Mercury at its greatest elongation soars higher up in the sky before sunrise at southerly latitudes, showcasing the Southern Hemisphere’s best morning apparition of Mercury for the year.

If you live at mid-northern latitudes, don’t give up entirely on spotting Mercury before sunrise. Get up before dawn to view the other three morning planets: Jupiter, Mars and Saturn. A line from Jupiter through Saturn and down to the horizon shows Mercury’s approximate rising point on the horizon. Scan this spot on the horizon with binoculars about 45 to 60 minutes before sunup, and you just might catch Mercury, the most elusive planet of the early morning sky.

Diagram of orbits of Mercury and Earth showing angles of view.
An inferior planet – a planet that orbits the sun inside of Earth’s orbit – appears in the evening sky at its greatest eastern elongation, and in the morning sky at its greatest western elongation. The 2 inferior planets are Mercury and Venus, residing at a mean distance of 0.387 and 0.723 astronomical units from the sun, respectively.

Bottom line: Mercury, although sitting way below the planets Mars, Jupiter and Saturn in the morning sky, nonetheless swings out to its farthest angular distance from the sun for the year on March 24, 2020. It’s the best morning apparition for the Southern Hemisphere, worst for the Northern Hemisphere. Looking ahead, Mercury will reach its maximum elongation in the evening sky on June 4, 2020 (23.6 degrees east of the sun).

March 23, 2020
Sky Archive

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Bruce McClure

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