Feb21-2021-at-8-pm-Moon-in-the-Winter-Hexagon
Moon in the Winter Hexagon (Chris Vaughan, Starry Night Education)

This Week’s Sky: February 15 to 21, 2021

It’s the Moon’s time to shine, as through the week it passes Uranus, Mars, the Winter Hexagon and Messier 35.

Wednesday, February 17 – Crescent Moon helps locate Uranus (evening)

Crescent Moon helps locate Uranus (Chris Vaughan, Starry Night Education)

Although a nearby bright Moon does magnitude 5.8 planet Uranus no favours, its monthly visits can help to show you where the distant planet is located. In the western sky on Wednesday, February 17, the 33-per-cent-illuminated crescent Moon will shine several finger widths to the upper left (or 4 degrees to the celestial southeast) of Uranus. Note Uranus’ position about midway between the medium-bright stars Menkar (Alpha Ceti) and Sheratan (Beta Arietis) and seek out the blue-green planet with binoculars, or even your unaided eyes, on a subsequent moonless night.

Thursday, February 18 – Moon meets Mars (evening)

Moon meets Mars (Chris Vaughan, Starry Night Education)

In the southwestern sky on the evening of Thursday, February 18, the waxing, half-illuminated Moon will be positioned several finger widths below (or 3.7 degrees to the celestial south) of Mars. The Moon and the planet will appear together in the field of view of binoculars (red circle). By the time they set in the west shortly after midnight local time, the diurnal rotation of the sky will lift the Moon to the planet’s left.

Friday, February 19 – First quarter Moon (at 18:47 GMT) passes Taurus

First quarter Moon (Chris Vaughan, Starry Night Education)

When the Moon completes the first quarter of its orbit around Earth at 1:47 p.m. EST (or 18:47 GMT) on Friday, February 19, the relative positions of the Earth, Sun and Moon will cause us to see it half-illuminated on its eastern side. At first quarter, the Moon always rises around noon and sets around midnight, so it is also visible in the afternoon daytime sky. The evenings surrounding first quarter are the best for seeing the lunar terrain when it is dramatically lit by low-angled sunlight. This first quarter phase will occur while the moon is passing several degrees to the right (celestial north) of the triangular face of Taurus, which is composed of the large Hyades star cluster and the bright foreground star Aldebaran. Look for the bright Pleiades star cluster to the Moon’s right.

Saturday, February 20 – The Lunar Straight Wall (evening)

Lunar Straight Wall (Chris Vaughan, Starry Night Education)

On Saturday evening, February 20, the pole-to-pole terminator boundary that divides the lit and dark hemispheres of the waxing gibbous Moon will fall just to the left (or lunar west) of Rupes Recta, also known as the Lunar Straight Wall. This feature is very obvious in good binoculars and backyard telescopes. The rupes, Latin for “cliff”, is a north-south-aligned fault scarp that extends for 65 miles (110 km) across the southeastern part of Mare Nubium — that’s the large dark region in the lower third of the Moon’s Earth-facing hemisphere. The Straight Wall is always prominent a day or two after first quarter, and again just before third quarter. For reference, the Straight Wall is located due north of the prominent crater Tycho.

Sunday, February 21 – Moon in the Winter Hexagon (evening)

Moon in the Winter Hexagon (Chris Vaughan, Starry Night Education)

The Winter Hexagon, also known as the Winter Football and Winter Circle, is an asterism composed of the brightest stars in the constellations of Canis Major, Orion, Taurus, Auriga, Gemini, and Canis Minor — specifically Sirius, Rigel, Aldebaran, Capella, Castor & Pollux and Procyon. After dusk, the huge pattern will stand upright in the southeastern sky, extending from 30 degrees above the horizon to overhead. The Milky Way passes vertically through the asterism. The hexagon is visible during evenings from mid-November to spring every year. The waxing gibbous Moon will travel through the asterism from February 20 to 22.

Sunday, February 21 – Bright Moon crosses Messier 35 (overnight)

Bright Moon crosses Messier 35 (Chris Vaughan, Starry Night Education)

On Sunday night, February 21, the eastward orbital motion of the Moon (green line) will carry it towards and across the large open star cluster known as Messier 35, or the Shoe-Buckle Cluster. After dusk on Sunday, the waxing gibbous Moon will be positioned several finger widths to the lower right of the cluster. Hour by hour, the Moon will approach the cluster. By the time the Moon sets after 3 a.m. local time, observers in the Eastern Time Zone will see the Moon only a finger’s width below the cluster. Observers farther west will see the Moon pass across the cluster and leave it behind before moonset. To best see Messier 35’s stars, hide the bright Moon beyond the upper edge of your binoculars’ field of view.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.

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