See what's up in the night sky for May 2021, including stargazing events and the moon's phases, in this Space.com gallery courtesy of Starry Night Software.
Monday, May 3 — Third quarter moon (1950 GMT)
The moon will officially reach its third quarter phase at 3:50 p.m. EDT (1950 GMT) on Monday, May 3. At third quarter our natural satellite always rises in the middle of the night and remains visible in the southern sky all morning. The moon will appear half-illuminated, on its western side — toward the predawn sun. Third quarter moons are positioned ahead of the Earth in our trip around the sun. About 3.5 hours later, Earth will occupy that same location in space. The ensuing week of moonless evening skies will be ideal for observing deep sky targets.
Tuesday, May 4 — Moon between Jupiter and Saturn (predawn)
When the third quarter moon rises over the southeastern horizon at about 3 a.m. local time on Tuesday, May 4, it will be positioned below and between bright Jupiter on the moon's left (or celestial northeast), and dimmer Saturn to the moon's upper right (or celestial northwest). The trio will make a lovely wide-field photograph when composed with some interesting landscape scenery.
Wednesday, May 5 — May is the month of Mercury (after sunset)
After sunset throughout the month of May, 2021 Mercury will be easily visible by Northern Hemisphere observers while it travels on the high side of a nearly vertical evening ecliptic plane. The speedy planet will climb away from the sun until mid-month, when it will be at peak visibility. In the second half of the month Mercury will descend sunward, passing much brighter and slower Venus near month-end. As the sky grows darker each night, look for Mercury low in the west-northwest as a white point of light shining some distance above Venus. Since the planet will be traversing the space between us and the sun, telescope views of Mercury in May will show that its disk is growing larger while waning in illuminated phase (inset).
Thursday, May 6 — Eta Aquarid meteor shower peaks (predawn)
The annual Eta Aquarid meteor shower is produced by particles of material left behind by repeated passages of Halley's Comet. The shower, which runs from April 19 to May 28, will peak in intensity before dawn on Thursday, May 6. Aquarids meteors will appear to be travelling away from a radiant point in Aquarius. That spot will lie near the southeastern horizon, not far from Jupiter. The southerly radiant makes this shower better for observers at low latitudes. On the peak night, watch for up to a few dozen meteors per hour, including some fireballs. A 25%-illuminated, waning crescent moon in the sky on the peak morning will reduce the number of meteors seen.
Friday, May 7 — The Virgo Cluster of galaxies (all night)
The Virgo Cluster contains as many as 2,000 galaxies — dozens of which are visible with amateur telescopes under dark sky conditions during spring evenings. The cluster spans nearly a fist's diameter (8 degrees) of sky on the border between Virgo and Coma Berenices. The brightest member is the elliptical galaxy Messier 49, which is located a generous palm's width to the lower right (or 8.5 degrees to the celestial southwest) of the bright star Vindemiatrix (Epsilon Virginis). Using low magnification, aim your telescope mid-way between Vindemiatrix and the bright star Denebola (Beta Leonis). That region contains a large number of bright galaxies, including the Messier objects M84 and M86 through M91. Markarian's Chain is a spectacular, curved line of galaxies spanning 1.5 degrees. It arcs to the upper left (celestial north) from M84.
Saturday, May 8 — Whirlpool and Pinwheel Galaxies (all night)
On evenings during May, the Big Dipper is positioned nearly overhead in the northeastern sky. Under dark sky conditions two impressive galaxies can be seen in binoculars (red circle) and backyard telescopes. You can use the bright star Alkaid at the tip of the dipper's handle to locate them. The Pinwheel Galaxy, or Messier 101, is a spectacular face-on spiral galaxy positioned a palm's width to the lower left (or 5.5 degrees to celestial north) of Alkaid, forming an equilateral triangle with Mizar, the double star at the bend of the handle. This relatively close galaxy (21 million light-years away) is nearly as large as the full moon in the sky (inset). Since the galaxy's light is spread over such a large area, its overall brightness is low. Aim your binoculars several finger's widths above Alkaid to discover the iconic Whirlpool Galaxy, aka Messier 51. This spiral galaxy's angular size is smaller, but it will look somewhat brighter in your binoculars and telescope (inset). A secondary galaxy core designated NGC5195 is linked to Messier 51 by a stream of stars.
Tuesday, May 11 — New moon (1900 GMT)
The moon will officially reach its new phase on Tuesday, May 11 at 3 p.m. EDT (1900 GMT). While new, the moon is travelling between Earth and the sun. Since sunlight can only reach the far side of the moon, and the moon is in the same region of the sky as the sun, the moon becomes completely hidden from view from anywhere on Earth for about a day. After the new moon phase Earth's celestial night-light will return to shine in the western evening sky.
Wednesday, May 12 — Crescent moon kisses Venus (after sunset)
Low in the west-northwestern sky after sunset on Wednesday, May 12, the young crescent moon will be positioned a finger's width to the lower left (or 1 degree to the celestial south) of Venus. The very bright planet will be easier to find first as the sky darkens after about 8 p.m. local time. Be sure that the sun has completely set before using binoculars or a telescope (red circle) to view the duo. Observers in much of New Zealand, Eastern Polynesia, and Easter Island can see the moon occult Venus in the mid-day sky.
Thursday, May 13 — Crescent moon near Mercury (after sunset)
A day after passing Venus, the crescent moon will climb to sit several finger widths to the left (or 3 degrees to the celestial southeast) of Mercury in the west-northwestern sky of Thursday, May 13. The pair will set at about 10 p.m. local time — well after sunset, allowing them to shine in a darkened sky. The moon and the planet will fit together in the field of view of binoculars (red circle). Bright Venus will shine below them.
Saturday, May 15 — Crescent moon and Mars (evening)
In the western sky after dusk on Saturday, May 15, the pretty, waxing crescent moon will shine several finger widths to the lower right (or 2 degrees to the celestial northwest) of the reddish planet Mars. The duo will appear together in binoculars (red circle). The moon's orbital motion will carry it closer to Mars by the time they set together shortly before midnight local time.
Sunday, May 16 — Moon occults bright star Kappa Geminorum (early evening)
On Sunday evening, May 16, observers in the eastern continental U.S. and Canada can watch the waxing gibbous moon occult the medium-bright star Kappa Geminorum (or κ Gem). The dark leading edge of the moon will cover the star first, causing its point of light to wink out. The star will reappear from behind the bright eastern limb of the moon some time later. Ingress and egress times vary by latitude — so use Starry Night or another astronomy app to look up the times where you live. In New York City, Kappa Geminorum will disappear at 9:37 p.m. EDT and re-appear at 10:39 p.m. EDT. Wherever you are observing from, start watching a few minutes before the appointed times. The event will be visible in binoculars and backyard telescopes — but remember that a telescope (red circle) will likely invert and/or mirror the scene shown here.
Sunday, May 16 — Mercury at greatest eastern elongation (after sunset)
In the west-northwestern sky on the evening of Sunday, May 16, Mercury (orbit shown in red) will be hours from its widest separation, 22 degrees east of the sun. With Mercury positioned above (north of) the evening ecliptic (green line), this appearance of the planet will offer excellent views for Northern Hemisphere observers, but not so for observers in the Southern Hemisphere. The optimal viewing period at mid-northern latitudes will be 8:30 p.m. to 9:30 p.m. local time. Viewed in a telescope (inset) the planet will exhibit a waning, half-illuminated phase.
Tuesday, May 18 — "Lunar X" in twilight (peaks at 9 p.m. EDT)
Several times a year, for a few hours near its first quarter phase, a feature on the moon called the "Lunar X" becomes visible in strong binoculars and backyard telescopes. When the rims of the craters Purbach, la Caille, and Blanchinus are illuminated from a particular angle of sunlight, they form a small, bright X-shape. The Lunar X is located on the terminator, about one third of the way up from the southern pole of the Moon (at 2 degrees east, 24 degrees south). The "X" is predicted to peak in intensity at around 9 p.m. EDT on Tuesday, May 18. That will be during waning daylight for observers in the eastern Americas — but you can observe the moon in a telescope during daytime, as long as you take care to avoid the sun. The Lunar X will be visible anywhere on Earth where the moon is shining, especially in a dark sky, between 2300 GMT on May 18 and 0300 GMT on May 19.
Wednesday, May 19 — First quarter moon (1912 GMT)
When the moon completes the first quarter of its orbit around Earth at 3:12 p.m. EDT (1912 GMT) on Wednesday, May 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-angle sunlight.
Sunday, May 23 — Double shadow transit on Jupiter (1517-1619 GMT)
From time to time, the small round black shadows cast by Jupiter's four Galilean moons become visible in amateur telescopes as they cross (or transit) the planet's disk. Before dawn on Sunday, May 23, observers located in Japan, New Zealand and Australia can see two of those shadows on Jupiter at the same time. At 1517 GMT (12:17 a.m. JST), Callisto's large shadow will join Io's smaller shadow already in transit. The two shadows will cross Jupiter together for an hour until Io's shadow moves off the planet at 1619 GMT. Callisto's shadow will continue to transit Jupiter until 2000 GMT.
Sunday, May 23 — Saturn stands still (predawn)
On Sunday, May 23, Saturn will cease its regular eastward motion through the distant stars of Capricornus and begin a retrograde loop (red curve with dates/times) that will last until mid-October. The apparent reversal in Saturn's motion is an effect of parallax produced when Earth, on a faster orbit, passes the Ringed Planet on the "inside track." You can observe the planet's direction change by noting how Saturn's distance from the nearby bright star Theta Capricornus varies over several days.
Wednesday, May 26 — Full milk supermoon (1114 GMT)
The moon will reach its full phase at 7:14 a.m. EDT (1114 GMT) on Wednesday, May 26. May's full moon always shines in or near the stars of Libra or Scorpius. Every culture around the world has developed its own stories about the full moon, and has assigned special names to each full moon. The indigenous Ojibwe groups of the Great Lakes region call the May full moon Zaagibagaa-giizis "Budding Moon" or Namebine-giizis, the "Sucker Moon." For them it signifies a time when Mother Earth again provides healing medicines. The Cree of North America call it Athikipisim, the "the Frog Moon" — the time when frogs become active in ponds and swamps. The Cherokee call it Ahnisguti, the "the Planting Moon," when the fields are plowed and sown. In European cultures, the moon is commonly called the Full Milk Moon, Full Flower Moon or Full Corn Planting Moon. When fully illuminated, the moon's geology is enhanced — especially the contrast between the bright, ancient, cratered highlands and the darker, younger, smoother maria. This full moon will occur nine hours after perigee, the point in the moon's orbit when it is closest to Earth, making this the largest supermoon of 2021.
Wednesday, May 26 — Total lunar eclipse (1111-1126 GMT)
The full moon of Wednesday, May 26 will also generate a total lunar eclipse. Due to the enlarged supermoon effect, this eclipse will barely qualify as a total one. When at maximum eclipse, the northern limb of the moon will be a scant 0.3 arc minutes from the northern edge of the umbra — limiting the length of totality to just 14.5 minutes. Observers should expect the southern half of the moon, which will extend much deeper into the umbra, to look noticeably darker than the northern half. The moon will first contact the umbra at 5:44:57 a.m. EDT (0944 GMT ) and will last touch the umbra three hours later at 8:52:22 a.m. EDT (1252 GMT). The entire eclipse will be visible across the Pacific Ocean, and from New Zealand and eastern Australia. Except for the northeastern states and provinces, observers in the Americas will see at least the initial stages of this eclipse. The moon will set before the eclipse ends for observers on the west coasts. Lunar eclipses are completely safe to observe unfiltered with your unaided eyes, binoculars, and telescopes.
Friday, May 28 — Double shadow transit on Jupiter (21:29-22:18 GMT)
From time to time, the small round black shadows cast by Jupiter's four Galilean moons become visible in amateur telescopes as they cross (or transit) the planet's disk. Before dawn on Sunday, May 23, observers located in Central Asia can see two of those shadows on Jupiter at the same time. At 5:29 p.m. EDT (2129 GMT), Io's small shadow will join Ganymede's large shadow already in transit. The two shadows will cross Jupiter together for 45 minutes until Ganymede's moves off the planet at 6:18 p.m. EDT (2218 GMT). Io's shadow will continue to transit Jupiter until 7:46 p.m. EDT
Friday, May 28 — Mercury kisses Venus (after sunset)
On the evenings surrounding Friday, May 28, speedy Mercury's descent sunward (red curve) will bring it very close to much brighter Venus. At their closest approach on Friday, the two planets will be separated by only 25 arc minutes — close enough to appear together in the eyepiece of a backyard telescope (red circle). (Don't point optical aids in that area of sky until after the sun has completely set.) Start looking for magnitude -3.85 Venus above the west-northwestern horizon after about 8:30 p.m. local time. Once the sky darkens sufficiently, magnitude 2.26 Mercury, 280 times dimmer than Venus, will appear to Venus' left (or celestial south).
Monday, May 31 — Gibbous moon near Saturn and Jupiter (predawn hours)
When the waning gibbous moon rises about an hour after midnight on Monday morning, May 31, it will be positioned less than a palm's width below (or 5 degrees to the celestial south of) Saturn. Jupiter will be shining brightly off to the pair's left. By sunrise, the trio will be in the lower part of the southern sky — making a lovely photo opportunity when composed with some interesting foreground scenery.