Moon, Mars, Uranus January 19, 20, 21

These next several evenings – January 19, 20 and 21, 2021 – watch for the waxing moon to sweep into the neighborhood of the bright planet Mars and and the faint planet Uranus, the 4th and 7th planets outward from the sun, respectively. (Incidentally, the 5th and 6th planets from the sun – Jupiter and Saturn – are not visible at present, because they sit too close to the afterglow of sunset.)

The moon will reach its first quarter phase on January 20, at 21:02 UTC. For us in North America, that means the first quarter moon comes during the daytime hours on January 20, at 4:02 p.m. EST, 3:02 p.m. CST, 2:02 p.m. MST and 1:02 p.m. PST. Guess what? You can still see the moon at this time from North America, because the first quarter moon will be above your horizon.

Want to find out when the moon rises and reaches its first quarter phase in your part of the world? Click on the Sunrise Sunset Calendar, remembering to check the Moon phases and Moonrise and moonset boxes.

The red planet Mars, though dimming over the last few months, still shines on 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.

The good news is that Mars and Uranus are close together on the sky’s dome, so Mars and Uranus will both take stage in a single binocular field of view for the next week or so. At conjunction, Mars passes 1 3/4 degrees to the north of Uranus on January 21, 2021, at about 24 hours UTC. (For reference, the width of your finger at an arm length approximates 2 degrees.) At North American time zones, that translates to January 21, at 7 p.m EST, 6 p.m. CST, 5 p.m. MST and 4 p.m. PST.

Image via Wikipedia. A quarter moon takes place whenever the sun-Earth-moon angle makes a right angle in space, with our planet Earth being at the vertex of this right angle.

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-sun making a 90-degree angle in space, with Earth at the vertex of this right angle. (See 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.

Mars at quadrature.

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-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.

By definition, and in the language of astronomy, the moon at its first quarter phase is at east quadrature – 90 degrees east of the sun in geocentric ecliptic longitude. Technically, the first quarter moon is not exactly 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.

Over the next several evenings – January 19, 20 and 21, 2021 – let the moon show you the red planet Mars, and then use Mars to find the distant ice giant planet, Uranus.

Bruce McClure