Elusive elongation

Mercury will on July 22 achieve its greatest elongation for this year.

Elongation – angular distance from the Sun – is the clue to the times when we can try to see the elusive little planet, which is the nearest to the Sun in linear distance and most often hidden in the glare. But elongation is a tricky clue, as illustrated in this graph from page 108 of Astronomical Calendar 2024.

Swinging around the Sun, Mercury makes three appearances to the east of it, that is, into our evening sky (the gray areas), and four to the west, our morning sky (the blue areas). And, of all these, the July 22 peak of elongation is the highest, 26.9°.

But that is not the visible height Mercury reaches above the horizon, which is indicated by the two other curves. The thick black curve is Mercury’s altitude at sunset or sunrise for places at latitude 40° north on Earth, such as much of America and Europe. The thin black line is for latitude 35° south, which will serve as an approximation for Australia, New Zealand, and South Africa.

The southern latitude is favored, on most though not all of the occasions. This time, Mercury does, for the south, reach a sunset altitude (24.1°) almost as great as its elongation, but for the north it attains only 14.5°.

Let’s look at the difference.

Here is the scene for a North American location on the evening nearest to the instant of maximum eastern elongation (which is July 22 7h Universal Time).

See the end note about enlarging illustrations. Arrows through the moving bodies show their movement (against the starry background) from 2 days earlier to 2 days later. The Sun is exaggerated 2 times in size.

As the altitude scale shows, at this time and location Mercury is 4° above the horizon. It is shining at magnitude 0.5, like the brightest stars. Regulus, slightly dimmer at magnitude 1.4 but higher, can help in finding it.

And the periodic comet bearing the name of Olbers, who asked a deep question about the starry sky, may be still at about 8 or perhaps one magnitude brighter, worth searching for with binoculars.

Here is the starkly contrasting scene for southerners.

Because Mercury is almost vertically above the Sun, it stands about 12° higher, taking almost full advantage of the large elongation.

The cause of the variation in Mercury’s performance is the peculiarity of its orbit, which, among all the major planets, departs most from circular (its eccentricity us 0.2) and from the plane of the ecliptic (inclination 7°).

If, when we look toward Mercury at its outward swing from the Sun, it is near aphelion, the “away-from-Sun” point of its orbit, it appears farther out.

And the aphelion is in the part of Mercury’s orbit that is south of the southern part of the ecliptic.

This space view, from 10° north of the ecliptic plane, shows the path of Mercury in July 2024, with a sightline from Earth to Mercury at the eastward elongation on 2024 July 22. You can see that the planet’s orbit is eccentric (differing from the circle of its mean distance) and inclined (tilted from the ecliptic, the plane of Earth’s orbit.

Elongation of time?

The day has, for millions of years, been growing milliseconds longer, because Earth’s rotation is gradually slowed by the tidal drag of the Moon on, mainly, our oceans, and mainly our fat “belt” of water around the equator.

Scientists have now measured the small but accelerating rate at which this process is being boosted by the rapid melting of ice, mainly the Greenland and Antarctic ice sheets. Water imprisoned in them is released, to swell the oceans. There may even be some disruption to the timekeeping systems that depend on the exact length of the day and on which our increasingly electronic society depends.

It is a subtle but massive indicator of human influence on the world’s climate.

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