What’s Up: Look for Mercury in the Western Sky

by | Sep 10, 2021 | Daily Space, Mercury, Sky Watching | 0 comments

What’s Up: Look for Mercury in the Western Sky
IMAGE: Mercury above the western horizon. CREDIT: Erik Madaus/Stellarium

This week on What’s Up is the greatest eastern elongation of Mercury. What that means is that Mercury will appear as far from the Sun in the sky as it ever gets when viewed from Earth. This is an important event for astronomers who want to observe the innermost planet as its close proximity to the Sun makes it difficult to see it in the Sun’s glare. In absolute terms, Mercury will reach almost 27 degrees away from the Sun, or about three fists away if you hold your hand out in front of you.

Mercury will be visible after sunset in the western sky for about an hour on September 13, this coming Monday. Just as the sun sets, look west, three fists up, and a little down. Yes, that’s confusing, but we have maps on our website, Daily Space.org, to help you out.

Although Mercury is often in the sky, it’s typically not visible from the Earth because of the Sun’s glare. However, the farther away from the Sun that Mercury gets, from the Earth’s point of view, the easier and safer it is to view. As a general rule, do not look directly at the Sun and especially please don’t look at it with any optics, such as binoculars or a telescope, that are not properly fitted with a solar filter.

This coming week, Mercury will be bright, magnitude 0.1, which is brighter than almost all the stars, and will be visible even from an urban environment. Although Mercury will be visible to the unaided eye, binoculars will provide a better view and will even show the phase of the planet. Yes planets, like the moon, have apparent phases! Mercury will appear partially dark like the Moon when it’s a few days away from being full. At eastern elongation, Mercury will take on a waxing gibbous phase, and then get more and more full each day until it’s a completely illuminated disc.

IMAGE: Not to scale. Mercury’s mean distance from the sun is about 0.39 times Earth’s distance from the sun. We’re looking down at the north side of the solar system plane. In this view, Mercury and Earth circle the sun in a counterclockwise direction. Earth and Mercury also rotate counterclockwise on their axes. At its greatest eastern elongation, Mercury is seen in the west after sunset; and at its greatest western elongation, Mercury is seen in the east before sunrise. CREDIT: EarthSky

Mercury is hard to see for another reason, the apparent line all planets trace, the ecliptic. This is an imaginary plane drawn out into space from the Earth’s equator, and all of the planets in the solar system appear to orbit in this plane. Mercury is no different. The angle between the ecliptic and the horizon changes depending on whether you are viewing in the morning or the evening. In the evening in the autumn, objects on the ecliptic are very low on the horizon, making them harder to view. The ecliptic is at its highest angle during an equinox, where the Sun’s light hits the Earth at a straight angle, which also explains why this particular event, in general, and Mercury in particular, is so much better observed from the southern hemisphere than up here in the northern hemisphere.

Right now, the best time to view Mercury in the northern hemisphere is its western elongation which happens in the morning starting next month and continuing into November, peaking in late October.

Mercury is a rather unusual planet. For years, planetary scientists had assumed that Mercury would be full of refractory elements – that is, stuff that doesn’t melt or boil easily. Instead, NASA’s MESSENGER mission revealed a surface with a lot of volatile elements, such as sodium. There is even evidence for water in craters near the poles of Mercury, similar to water in the permanently shadowed craters on the Moon.

Geochemists had also assumed that Mercury’s surface would be high in iron, due to the large size of Mercury’s core and its close proximity to the Sun. Instead, the surface of Mercury appears to be especially low in iron compared to what we see on other planets.

How can this be? Well, it may be because the chemistry of Mercury is especially reducing, meaning that the iron wouldn’t be incorporated into rocks as it can be on the Earth or Mars, but instead, it forms as metallic iron, sinking into the core. Whatever the case, Mercury presents a large number of apparent contradictions that planetary scientists are still working to understand.

Get out there, look up – but not too high up – enjoy Mercury, and know that it is a weird world we still don’t understand.

More Information

Mercury at greatest elongation east (In-The-Sky.org)

Mercury at greatest elongation west (In-The-Sky.org)

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