Moon, Mars, Uranus January 19, 20, 21

On January 19, 20 and 21, 2021, watch for the waxing moon to sweep near the bright planet Mars and the exceedingly faint planet Uranus. Mars is the fourth planet – and Uranus the seventh planet – outward from our sun. Mars and Uranus appear so close together on the sky’s dome now that – for the coming week or so – you could see them in a single binocular field of view, if the moon weren’t in the way. Mars will pass 1.75 degrees to the north of Uranus on January 21. More about that below.

EarthSky’s lunar calendar shows the moon phase for every day in 2021. Order yours before they’re gone!

The moon will reach its first quarter phase on January 20, at 21:02 UTC. For us in North America, that means the exact moment of first quarter moon comes during our daytime hours on January 20 (4:02 p.m. EST, 3:02 p.m. CST, 2:02 p.m. MST and 1:02 p.m. PST). But guess what? You can still see the moon from North America at this time, because first quarter moons tend to rise around noon and set around midnight. Thus this month’s first quarter moon will be above our North American horizons at the crest of this particular phase. Look for it – pale and dreamlike – against the blue sky. Then look for the moon near Mars when night falls!

A first quarter moon, with Uranus and Mars nearby.
View at EarthSky Community Photos. | Nice catch from Helio C. Vital in Rio de Janeiro, Brazil! Very faint Uranus and brighter Mars are now in a single binocular field, but Uranus would have been difficult to see on the night of January 20, 2021, so near the moon’s glare. Helio said he reduced glare and enhanced contrast using PhotoScape. Thank you, Helio!

Want to find out when the moon rises and reaches its first quarter phase in your part of the world? Visit Sunrise Sunset Calendars, remembering to check the moon phases and moonrise and moonset boxes.

Incidentally, the fifth and sixth planets from the sun – Jupiter and Saturn – are also still, officially, in the evening sky. They’re there, but not visible at present, because they sit too close to the afterglow of sunset.

And there’s one more planet you should still be able to see after sunset. And that’s the first planet from the sun, Mercury. Its greatest elongation – or greatest apparent distance from the sun for this evening apparition – will come on January 23-24.

Sky chart with nearly vertical green line of ecliptic and Fomalhaut and Mercury near twilit horizon.
Mercury will be reaching the outer edge of its orbit as seen from Earth on January 23-24, 2021, placing Mercury optimally high in the western evening sky. You should be able to see it now, for for a shoert while after sunset. What’s more, Mercury shines more brilliantly than almost any star in the starry sky and nearly 5 times brighter than the nearby star Fomalhaut, a respectable 1st-magnitude star. Most likely, if you only see one starlike object near the sunset point on the horizon as dusk ebbs into darkness, it’s Mercury. Read more.

Mars and Uranus in a single binocular field. Now back to Mars and Uranus, which are high in the evening sky when night falls, as seen from around the globe.

The red planet Mars has dimmed over the last few months as Earth has been rushing along ahead of it in our smaller, faster orbit around the sun. But Mars still shines on a par with the sky’s brightest stars. Given clear skies, you should have little trouble viewing Mars as that brilliant ruddy “star” in the moon’s vicinity.

Uranus, on the other hand, is quite faint, well over 150 times fainter than Mars. Uranus is said to be the outermost of the sun’s planets visible with the eye alone. But seeing it with the eye requires a very dark sky, and probably no moon (certainly no nearby moon).

The interesting news is that Mars and Uranus are close together on the sky’s dome, so that – theoretically – you could see Mars and Uranus in a single binocular field of view for the next week or so, if the moon weren’t in the way. Mars will pass 1.75 degrees to the north of Uranus on January 22, 2021, at about 0 hours UTC. For reference, the width of your finger at arm’s length approximates 2 degrees.

Unluckily for us, just as Mars and Uranus are closest, the moon will be nearly on top of them! Bright moonlight will make faint Uranus hard to see, even with an optical aid.

Diagram with Earth in center and moon phases around it in a circle.
A quarter moon happens whenever the sun-Earth-moon angle makes a right angle in space, with our planet Earth being at the vertex of this right angle. Image via Wikipedia.

At first quarter phase, the moon is said to be at eastern quadrature, because the moon at this juncture lies 90 degrees east of the sun on the sky’s dome.

In fact, if you could look down upon the Earth and moon at the moon’s first quarter phase (eastern quadrature), you’d see the moon, Earth and sun making a 90-degree angle in space, with Earth at the vertex of this right angle, as shown in the diagram above.

Superior planets – planets that revolve around the sun outside of Earth’s orbit – also reach eastern quadrature in Earth’s sky at regular intervals. In next next few weeks, Uranus and then Mars will reach eastern elongation (90 degrees east of the sun) as well. Uranus will be at eastern elongation on January 26, 2021, at 12:48 UTC, and Mars on February 1, 2021, at 10:34 UTC. Once again, if you could look down upon the plane of the solar system, you’d see the superior planet-Earth-sun making a right angle in space, with Earth at the vertex of this right angle.

Diagram with sun, Earth, and 2 positions of Mars in its orbit.
Bird’s-eye view of a superior planet at quadrature as seen from the north side of the solar system. From this perspective, Earth and Mars orbit the sun counterclockwise, and spin around their rotational axes counterclockwise. When Mars is at quadrature, the sun, Earth, and Mars make a right angle in space, with Earth residing at the vertex of this angle. Because Earth is an inferior planet as seen from Mars, Earth is at or near its greatest elongation from the sun.

Unlike the moon, the superior planets are not half-illuminated at quadrature. That’s because these superior planets are so much farther from Earth than our moon is. As seen through the telescope, Mars shrinks to its smallest phase at or around quadrature. Nonetheless, its disk still appears nearly 89% illuminated by sunshine.

Because Jupiter and Saturn are so distant as compared to the moon and Mars, the telescope always shows Jupiter and Saturn at or very close to 100% illuminated in Earth’s sky. Yet, quadrature presents the best time to view Jupiter’s moons being eclipsed by Jupiter’s shadow, or to see Saturn’s shadow angling across Saturn’s rings.

Diagram of Earth, moon, and sun making a right angle with text annotiations.
By definition, the moon at its first quarter phase is at east quadrature – 90 degrees east of the sun in ecliptic longitude. Technically, the first quarter moon is a tiny bit more than 50% illuminated at east quadrature, although the lunar disk certainly looks half lit to the eye. Depending on the month, the illuminated portion of first quarter moon can vary from 50.117% to 50.138%, and the time period between dichotomy (exactly half-illuminated) and quadrature can vary anywhere from about 15 to 21 minutes.

Bottom line: On the evenings of January 19, 20 and 21, 2021, let the waxing moon show you the red planet Mars. Then – when the moon moves away and you have a dark sky – use Mars to find the distant ice giant planet, Uranus.

January 19, 2021

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Bruce McClure

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