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The Sky This Week
U.S. Naval Observatory's Weekly Blog
A Lunar Eclipse...For Some of Us!
by Geoff Chester, USNO Public Affairs
18 May 2021
The Moon in HDR, imaged 2021 March 21, 00:55 UT from Alexandria, Virginia
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor, Canon EOS Rebel SL2 DSLR and 1.4X tele-extender.
HDR composite of 4 exposures, 1/250, 1/60, 1/15, & 1 seconds, ISO 400.
The Moon waxes toward her full phase this week, plunging southward along the ecliptic where she will meet up with the rising stars of summer. Full Moon occurs on the 26th at 7:14 am Eastern Daylight Time. May’s Full Moon is popularly known as the Flower Moon due to the abundance of wildflowers that bloom at this time of year. It is also known as the Milk Moon in ancient European traditions and as the Corn Planting Moon among several Native American cultures.
Early risers on the 26th will be able to see part or all of the first of two lunar eclipses, depending on their location. Here in the eastern US observers west of a line from Lake Ontario to the DelMarVa peninsula can see the Moon work her way into Earth’s penumbral shadow beginning at 4:46 am EDT. Luna’s eastern limb will begin to take on a greyish tint as she moves closer to the umbral shadow, which will occur at 5:44 am EDT. Unfortunately, here in Washington, Luna sets five minutes later! However, observers living farther west will get a progressively better show. Residents near the Front Range of the Rocky Mountains will see the Moon enter the total phase at 5:09 am MDT. The total phase of this eclipse only lasts about nine minutes, though, since the Moon passes very close to the northern edge of the umbral shadow. She will emerge from her partial phases for residents along the California coast at 5:52 am PDT. While most of us get short-changed by this eclipse, we will have a better view of the very deep partial lunar eclipse that will occur on November 19th. In that event 98 percent of the Moon’s disc will be in the umbral shadow, and all aspects of the eclipse will be visible from the entire nation.
The spring constellations are now prominent in the evening sky, but the growing Moon’s light begins to wash out all but the brightest of the season’s stars. Luna may be found northeast of the star Regulus, heart of Leo, the Lion, on the 19th, and by the 22nd she will have moved to the vicinity of Spica, brightest star in Virgo, goddess of the Harvest. Each of these constellations sport attractive double stars that will be near the Moon on those dates. North of the Moon on the 19th is the second-magnitude star Algieba, located in the Lion’s “mane”. Binoculars will show this star with a distinctive yellow cast. Just below it is the fifth magnitude star 40 Leonis. In a small telescope Algieba reveals itself as a fine pair of yellow-tinted suns nestled close together. The pair orbit their center of mass once every 500 years, and for the next century appear as widely separated as they can get. By the year 2321 they will be too close to resolve in most amateur telescopes. On the 22nd look a few degrees west of the Moon for another fine double star, Porrima. This pair consists of two nearly identical stars that orbit each other once every 169 years. The stars appeared closest together in 2005; at the time they were difficult to resolve in the Observatory’s 12-inch refractor. Since that time the gap between them has widened, and they are now easily split in my 4-inch telescope. This is one of the few pairs of stars that show perceptible changes in separation and position angle from year to year. Because the pair have such a well-studied orbit, we can deduce a number of their physical parameters.
The early evening sky continues to host Mercury and Venus in the twilight hour. Venus should be easy to spot if you have clear skies and a low western horizon. Half an hour after sunset on the 18th the dazzling planet will be about five degrees above the skyline, with much fainter Mercury about six degrees higher. Binoculars will help you locate Mercury in the twilight glow. By the end of the week Venus will have crept up on Mercury, while the latter begins his plunge toward the Sun. On the 25th they will be about three degrees apart. They will be closest together on the 28th, when they are a scant half-degree apart.
Mars is making his way eastward through the stars of Gemini, which can be seen prominently in the west as twilight ends. The red planet and the Twin Stars form a different triangle with each passing night. Mars will pass just over a degree north of the third-magnitude star Wasat on the 23rd and 24th.
Jupiter and Saturn are still best placed for viewing before sunrise, but they are making steady progress toward a fine evening show in the summer months. Saturn reaches the first stationary point in the current apparition on the 23rd. He will slowly begin to slink westward among the stars of Capricornus before resuming direct eastward motion in October. Jupiter shines down from the dim stars of Aquarius, dominating that part of the morning sky. He will begin his retrograde loop a month from now.
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