Astronomy Essentials

Mercury greatest morning elongation October 8

Mercury chart Sun at center with one ring around for inner planets and one for Earth showing greatest elongation.
Greatest elongation represents the best time for stargazers to view the inner planets, Mercury and Venus. Illustration via John Jardine Goss.

Mercury will become visible near the sunrise point around the last few days of September 2022. Mercury will be brightest before sunrise from early to mid-October.

When to watch: Beginning the last few days of September, start watching for Mercury in the eastern sky before sunrise. Greatest elongation – when Mercury will be farthest from the sunrise on our sky’s dome – is on October 8. Then afterwards the planet will continue brightening. So, later in October, although it’ll be edging back toward the sunrise, Mercury will be easier to see in the morning twilight.
Where to look: Look in the sunrise direction, as the sky is lightening.
Greatest elongation is at 21 UTC on October 8, 2022.
Note: As soon as you can spot Mercury – beginning around September 30 – notice that it brightens quickly reaching -0.6 magnitude by October . In fact, it will continue getting brighter reaching -1.1 magnitude near the end of October.

Northern versus Southern Hemisphere

Mercury in morning twighligh first half of October.
From the Northern Hemisphere, Mercury lies in the east before sunrise the first half of October. The little planet reaches its furthest angle from the sun on October 8. And that’s also, when it appears at its greatest distance from the sun reaching its highest position above the horizon before sunrise.
Mercury in morning twilight during first half of October.
From the Southern Hemisphere, Mercury peeks above the eastern horizon just before sunrise in the first half of October. Because the ecliptic – or path of the sun, moon and planets – is tilted sharply with respect to the horizon, the little planet doesn’t rise very high before sunrise.

At greatest elongation in October 2022

– Mercury’s distance from the sun on the sky’s dome is 18 degrees.
– Mercury shines at magnitude -0.6 at elongation, and will reach -1.1 magnitude until it slips away in the morning glare at the beginning of November.
– Through a telescope on October 8, Mercury appears 51% illuminated, in a waxing gibbous phase, and 7 arcseconds across. By the way, it will be almost 100% illuminated by the end of October.

For precise sun and Mercury rising times at your location:

Old Farmer’s Almanac (U.S. and Canada)
timeanddate.com (worldwide).
Stellarium (online planetarium program)

Mercury events in 2022 and 2023

Sep 23, 2022: Inferior conjunction (races between Earth and sun)
Oct 8, 2022: Greatest elongation (morning)
Nov 8, 2022: Superior conjunction (passes behind sun from Earth)
Dec 21, 2022: Greatest elongation (evening)
Jan 7, 2023: Inferior conjunction (races between Earth and sun)
Jan 30, 2023: Greatest elongation (morning)
Mar 17, 2023: Superior conjunction (passes behind sun from Earth)
Apr 11, 2023: Greatest elongation (evening)

Heliocentric view of Mercury October 2022

Circle with sun at center, planets around, and zodiac names on outer edge.
View larger. | Heliocentric view of solar system, October 2022. Chart via Guy Ottewell.

A comparison of elongations

Not all of Mercury’s greatest elongations are created equal. Some are greater than others. The farthest from the sun that Mercury can ever appear on the sky’s dome is about 28 degrees. The least distance is around 18 degrees.

Elongations are also better or worse depending on the time of year they occur. The October 2022 elongation is better from the Northern Hemisphere than the Southern Hemisphere. For instance, the evening apparition of Mercury on August 27, 2022, was challenging for the Northern Hemisphere, but it great for the Southern Hemisphere.

Chart with light blue and gray waves, black annotations, comparing Mercury elongatons in 2022.
View larger. | Mercury elongations compared. Gray areas represent evening apparitions (eastward elongation). Blue areas represent morning apparitions (westward elongation). The top figures are the maximum elongations, reached at the top dates shown beneath. Curves show the altitude of the planet above the horizon at sunrise or sunset, for latitude 40 degrees north (thick line) and 35 degrees south (thin). Maxima are reached at the parenthesized dates below (40 degrees north bold). Chart via Guy Ottewell’s 2022 Astronomical Calendar.

In the autumn for either hemisphere, the ecliptic – or path of the sun, moon and planets – makes a narrow angle to the horizon in the evening. But it makes a steep slant, nearly perpendicular, in the morning. So – in autumn from either hemisphere – morning elongations of Mercury are best. Then Mercury appears higher above the horizon and farther from the glow of the sun. Evening elongations in autumn are harder to see.

Bottom line: Watch for Mercury in the east before sunup in October 2022. Mercury reaches greatest elongation – its greatest distance from the sunrise – on October 8 and remains bright through the end of the month.

Posted 
October 2, 2022
 in 
Astronomy Essentials

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