Image of the moon and an elk

A setting Moon and elk at Mammoth Hot Springs in Yellowstone National Park in March 2015. Credit: National Park Service/Neal Herbert | Full image and caption

The Next Full Moon is the Buck Moon, Thunder Moon, Hay Moon, Mead Moon, Tu B'Av, Guru Purnima, Asalha Puha (also known as Dharma Day or Esala Poya), and the start of Vassa.

The next full Moon will be on Friday night, July 23, 2021, appearing opposite the Sun in Earth-based longitude at 10:37 p.m. EDT. While this will be on Friday for most of the Americas, from Newfoundland and Greenland eastward to the International Dateline this will be on Saturday, July 24, 2021. Many almanacs and commercial calendars are based on Coordinated Universal Time (UTC) and will show this full Moon on Saturday. The Moon will appear full for about three days around the peak of the full Moon, from Thursday evening through Sunday morning.

What's in a name

The Maine Farmer's Almanac first published Native American names for the full Moons in the 1930s. According to this almanac, as the full Moon in July – the Algonquin tribes in what is now the northeastern United States called this full Moon the Buck Moon. Early summer is normally when the new antlers of buck deer push out of their foreheads in coatings of velvety fur. They also called this the Thunder Moon because of early Summer's frequent thunderstorms.

Europeans called this the Hay Moon for the haymaking in June and July, and sometimes the Mead Moon (although this name and "Honey Moon" were also used for the previous full Moon). Mead is created by fermenting honey mixed with water, sometimes adding fruits, spices, grains, or hops.

For Hindus, Buddhists, and Jains, this is the Guru Full Moon (Guru Purnima), celebrated as a time for clearing the mind and honoring the guru or spiritual master.

For Theravada Buddhists, this full Moon is Asalha Puha, also known as Dharma Day or Esala Poya, an important festival celebrating Buddha's first sermon. As the full Moon day of Waso (the fourth month of the traditional Burmese lunisolar calendar), this is the start of the three-month annual Buddhist retreat called Vassa.

The Moon and calendars

In many traditional lunisolar calendars, full Moons fall in the middle of the lunar months. This full Moon is in the middle of the sixth month of the Chinese calendar and Av in the Hebrew calendar, corresponding with Tu B'Av, a holiday in modern Israel similar to Valentine's Day.

"As usual, the wearing of suitably celebratory celestial attire is encouraged in honor of the full Moon."
- Gordon Johnston

In the Islamic calendar, the months start with the first sighting of the waxing crescent Moon shortly after the New Moon. This full Moon is near the middle of Dhu al-Hijjah, the 12th and final month of the Islamic year. Dhu al-Hijjah is the month of the Hajj, the Festival of the Sacrifice, and is one of four sacred months during which fighting is forbidden.

Since this is the Thunder Moon, a quick note on lightning safety. Most of the lightning that strikes the ground arcs from the negatively charged bottom of the storm to the ground underneath the storm. Much rarer is positive lightning, which arcs from the top of a thunderstorm to strike the ground up to eight miles away. Positive lightning can sometimes strike areas where the sky is clear (hence the term "bolt out of the blue"). Because it arcs across a greater distance it tends to be 5 to 10 times more powerful than regular ground strikes. Because it can strike dry areas outside of the storm, positive lightning tends to start more fires than negative lightning. Although positive lightning is rare (less than 5% of all lightning strikes), the lack of warning combined with its greater power tends to make it more lethal. A good rule to follow is, if you can hear the thunder, you can be struck by the lightning.

As usual, the wearing of suitably celebratory celestial attire is encouraged in honor of the full Moon. Be safe (especially during thunderstorms), avoid starting wars, and take a moment to clear your mind.

Here is a summary of celestial events between now and the full Moon after next (with times and angles based on the location of NASA Headquarters in Washington):

As summer continues, the daily periods of sunlight continue to shorten. On Friday, July 23, 2021, (the day of the full Moon), morning twilight will begin at 4:54 a.m. EDT, sunrise will be at 6:02 a.m., solar noon will be at 1:14:35 p.m. when the Sun will reach its maximum altitude of 71.04 degrees, sunset will be at 8:27 p.m., and evening twilight will end at 9:35 p.m. By Sunday, Aug. 22, (the day of the full Moon after next), morning twilight will begin at 5:27 a.m., sunrise will be at 6:28 a.m., solar noon will be at 1:10:51 p.m. when the Sun will reach its maximum altitude of 62.66 degrees, sunset will be at 7:53 p.m., and evening twilight will end at 8:54 p.m.

Jupiter and Saturn return

We are coming up on what should be a great time for Jupiter and Saturn watching, especially with a backyard telescope. Saturn will be at its closest and brightest for the year on Aug. 2, 2021, while Jupiter will be at its closest and brightest on Aug. 19, (called "opposition" because they will be opposite the Earth from the Sun). Both will appear to shift toward the west over the coming months, making them visible earlier in the evening sky (and friendlier for backyard stargazing, especially if you have young ones with earlier bedtimes). With clear skies and a backyard telescope, you should be able to see Jupiter's four bright moons, Ganymede, Callisto, Europa, and Io, noticeably shifting positions in the course of an evening. For Saturn, you should be able to see Saturn's rings as well as Saturn's largest moon, Titan.

Two meteor showers coming

Two major meteor showers are expected to peak during this lunar cycle, but only one will have good visibility from our northern mid-latitudes (weather permitting). The Southern Delta-Aquariids should be active from around July 12 to Aug. 23, peaking on July 30. This will not be a good year for viewing these meteors for two reasons. They are best seen from the Southern Hemisphere and this year moonlight from the waning half Moon will interfere with the few meteors that may appear over our more northern location.

More promising is the Perseid meteor shower. This meteor shower is expected to be active from July 17 to Aug. 24, 2021, with a broad peak centered on the afternoon of Aug. 12 between 3 p.m. and 6 p.m. EDT (when we can't see them). These meteors appear to radiate out in all directions from the constellation Perseus (hence their name). This radiant is far enough north to be relatively high in the northeast after 10 or 11 p.m. Since this shower tends to have a broad peak you may be able to see meteors either the early morning of Aug. 12 or the early morning of Aug. 13. The International Meteor Organization's 2021 Meteor Shower Calendar indicates that past variations in the actual peak have tended to come after the predicted maximum. If this holds true this year, the best time to look from the East Coast may be from after moonset on Aug. 12 (at 10:40 p.m. for the Washington area) into the early morning of Aug. 13, ending when the first signs of dawn begin at about 4:40 a.m.

Under ideal conditions at its peak, the Perseids can produce about 100 visible meteors per hour, making it potentially one of the three best meteor showers of the year (the others being the Quadrantids in early January and the Geminids in mid-December). The Perseid meteor shower is caused by dust from the comet 109P/Swift-Tuttle entering our atmosphere at 132,000 miles per hour (59 kilometers per second), so fast that air cannot move out of the way fast enough, and gets compressed and heated until it glows white-hot.

The best conditions for viewing these meteors would be if the weather is clear with no clouds or high hazes, you go to a place far from any light sources or urban light pollution, and you have a clear view of a wide expanse of the sky. Be sure to give your eyes plenty of time to adapt to the dark. The rod cells in your eyes are more sensitive to low light levels but play little role in color vision. Your color-sensing cone cells are concentrated near the center of your view with more of the rod cells on the edge of your view. Since some meteors are faint, you will tend to see more meteors from the "corner of your eye" (which is why you need a view of a large part of the sky). Your color vision (cone cells) will adapt to darkness in about 10 minutes, but your more sensitive night vision will continue to improve for an hour or more (with most of the improvement in the first 35 to 45 minutes). The more sensitive your eyes are, the more chance you have of seeing meteors. Even a short exposure to light (from passing car headlights, etc.) will start the adaptation over again – so no turning on a light or your cell phone to check what time it is.

Evening Sky Summary

On the evening of Friday, July 23, 2021, (the day of the full Moon), as evening twilight ends (at 9:35 p.m. EDT), the brightest planet visible will be Venus, appearing as the Evening Star 5 degrees above the horizon in the west-northwest. The bright star Regulus will appear to the lower right of Venus. The third brightest planet in the sky will be Mars, appearing farther to the lower right of Venus only 1.5 degrees above the horizon (and setting about 10 minutes after evening twilight ends). The second brightest planet in the sky will be Saturn, appearing to the left of the rising full Moon in the east-southeast at 7 degrees above the horizon. The bright star closest to overhead will be Vega, appearing 66 degrees above the horizon in the east. Vega is the 5th brightest star in our night sky and the brightest of the three stars in the Summer Triangle. Vega is about twice as massive as our Sun, 40 times brighter, and about 25 light-years from us.

As the lunar cycle progresses, Saturn, Mars, and the background of stars will appear to shift toward the west each evening (although it is actually the Earth that is moving around the Sun toward the east). The bright planet Venus and the fainter planet Mars will appear to shift to the left along the horizon, with Venus shifting away from the bright star Regulus and Mars shifting toward Regulus and the horizon.

In late July 2021, Jupiter will begin appearing above the horizon in the east-southeast as evening twilight ends. Mars will appear to shift past the bright star Regulus and both Regulus and Mars will begin setting before evening twilight ends. In the second week of August, the planet Mercury will begin emerging from the glow of dusk about 30 minutes after sunset.

By the evening of Sunday, Aug. 22, 2021, (the day of the full Moon after next), as evening twilight ends at 8:54 p.m. EDT, the brightest planet visible will be Venus, appearing as the Evening Star 6 degrees above the horizon in the west. The next brightest planet will be Jupiter, appearing 12 degrees above the horizon in the east-southeast. The last (in brightness) of the visible planets in the sky will be Saturn, appearing 20 degrees above the horizon in the southeast. The planets Mercury and Mars will have set as evening twilight ends, but Mercury (and possibly the fainter planet Mars) might be visible low in the west from about 30 minutes after sunset (8:23 p.m.) until Mars sets 10 minutes later (at 8:34 p.m.), with Mercury setting about 7 minutes after Mars (at 8:41 p.m.). The bright star Vega will have shifted to appear nearly overhead, 81 degrees above the eastern horizon.

Morning Sky Summary

In the morning sky this time of year we begin to see the bright stars from the local arm of our home galaxy. This Orion Arm is about 3,500 lightyears across and 10,000 lightyears long. The bright stars we see in this arm (and their distance from us) include the three stars of Orion's Belt, Rigel (860 lightyears), Betelgeuse (548 lightyears), Polaris (about 400 lightyears), and Deneb (about 2,600 lightyears). We are on the inner edge of this arm, so when we are looking toward these stars we are looking away from the center of the galaxy. If you look about halfway between the stars Capella and Betelgeuse you are looking in the opposite direction from the galactic center.

On the morning of July 23, 2021, the day of the full Moon, as morning twilight begins at 4:53 a.m. EDT, the bright planet Jupiter will appear 34 degrees above the south-southwestern horizon with the fainter planet Saturn 20 degrees above the southwestern horizon. No particularly bright star will be appearing nearly overhead, with the closest being Deneb from the Summer Triangle, appearing about 55 degrees above the west-northwestern horizon. Mercury will not have risen yet but will rise in the east-northeast about 16 minutes later at 5:09 a.m., and may be visible for about 20 or so minutes before it is masked by the glow of dawn.

As the lunar cycle progresses, Jupiter and Saturn along with the background of stars will appear to shift toward the west, while Mercury will shift closer toward the Sun until it is lost in the glow of dawn in late July. By mid-August the planet Saturn will begin setting before morning twilight begins. By the morning of Sunday, Aug. 22, 2021, the day of the full Moon after next, as morning twilight begins at 5:27 a.m. EDT, the bright planet Jupiter will appear 9 degrees above the west-southwestern horizon, with the full Moon appearing to the left of Jupiter. No particularly bright star will be appearing near overhead, with the closest being Capella, appearing about 57 degrees above the east-northeastern horizon. Although Capella appears as a single star (the 6th brightest star in our night sky), it is actually two pairs of stars orbiting each other (a total of 4 stars). Capella is about 43 lightyears from us.

Daily Guide

Here is a more detailed, day-by-day listing of celestial events between now and the full Moon after next (again with times and angles based on the location of NASA Headquarters in Washington:

Late July

Even though they are not usually visible, I include in these Moon missives information about Near Earth Objects (mostly asteroids) that may pass the Earth within 5 lunar distances, because I find it interesting that we have discovered so many. Sometime in mid-to late-July, 2021 (2021-Jul-17 19:03 UTC with 4 days, 20 hours, 30 minutes uncertainty), Near-Earth Object (2019 NB7), between 29 to 65 feet (9 and 20 meters) across, will pass the Earth at between 1.7 and 39.3 lunar distances (nominally 15.2), traveling at 30,800 miles per hour (13.76 kilometers per second).

July 19-20

On Monday evening into Tuesday morning, July 19 to 20, 2021, the bright star Antares will appear about 8 degrees to the lower left of the waxing gibbous Moon. The pair will appear in the south as evening twilight ends at 9:39 p.m. EDT) and will set in the west-southwest at about the same time on Tuesday morning (around 2:15 a.m.).

By Wednesday morning, the Moon will have shifted such that Antares will appear about 8 degrees to the right of the Moon, with Antares setting first Wednesday morning at 2:10 a.m. EDT.

July 21

Early on Wednesday morning, July 21, 2021, at 1:32 a.m. EDT (2021-Jul-21 05:32 UTC), Near-Earth Object (2021 NB6), between 44 to 98 feet (13 and 30 meters) across, will pass the Earth at 2.4 lunar distances traveling at 12,300 miles per hour (5.52 kilometers per second).

Sometime during the week of July 21, 2021 (2021-Jul-21 09:48 UTC with 3 days, 1 hour, 6 minutes uncertainty), Near-Earth Object (2014 BP43), between 44 to 98 feet (13 and 30 meters) across, will pass the Earth at between 4.3 and 35.3 lunar distances (nominally 16.9), traveling at 18,900 miles per hour (8.46 kilometers per second).

Wednesday morning at 6:25 a.m. EDT, the Moon will be at perigee, its closest to the Earth for this orbit.

On Wednesday afternoon, at about 3:54 p.m. EDT (2021-Jul-21 19:54 UTC with 6 minutes uncertainty), Near-Earth Object (2021 NA8), between 50 to 112 feet (15 and 34 meters) across, will pass the Earth at 3.8 lunar distances traveling at 22,500 miles per hour (10.05 kilometers per second).

Wednesday evening, the bright planet Venus and the bright star Regulus will appear nearest each other, with Regulus 1 degree to the lower left of Venus. As evening twilight ends at 9:37 p.m. EDT, Venus will appear about 5 degrees above the west-northwestern horizon. The planet Mars will appear farther to the lower right at only 2 degrees above the horizon. Mars will set first (at 9:49 p.m.), Regulus next (at 10 p.m.), and Venus last (at 10:04 p.m.).

July 23

As mentioned above, the next full Moon will be Friday night, July 23, 2021, at 10:37 p.m. EDT. The Moon will appear full for about 3 days around this time, from Thursday night through Sunday morning.

Friday night into Saturday morning, July 23 to 24, 2021, the full Moon will shift toward the planet Saturn such that Saturn will appear about 8 degrees above the Moon in the southwest by the time morning twilight begins.

By Saturday evening, July 24, 2021, the Moon will have shifted such that Saturn will appear about 7 degrees above the full Moon, rising in the east-southeast as evening twilight ends. The Moon will appear to shift away from Saturn as Saturday night progresses into the morning of Sunday, July 25.

July 25-26

Sunday night into Monday morning, July 25 to 26, 2021, the waning gibbous Moon will appear to pass below the bright planet Jupiter. The Moon will rise before midnight at 10 p.m. EDT, with Jupiter appearing about 4 degrees to the upper left of the Moon. By the time twilight begins Monday morning at 4:57 a.m., the Moon will have shifted such that Jupiter will appear about 6 degrees to the upper right of the Moon.

July 28

Wednesday evening, July 28, 2021, will be the first evening the bright planet Jupiter will appear above the horizon in the east-southeast and the last evening the bright star Regulus will appear above the horizon in the west-northwest as evening twilight ends at 7:30 p.m. EDT.

July 29

Thursday, July 29, 2021, will be the evening when the planet Mars and the bright star Regulus will appear nearest each other low on the west-northwestern horizon. Regulus will set shortly before evening twilight ends and Mars will set shortly after evening twilight ends (at least for Washington, and similar latitudes).

July 30

Friday evening, July 30, 2021, will be the last evening the planet Mars will be above the horizon in the west-northwest as evening twilight ends.

July 31

Saturday morning, July 31, 2021, the waning Moon will appear half-full as it reaches its last quarter at 9:16 a.m. EDT.


August 1

Sunday, Aug. 1, 2021, is about halfway between the summer solstice and the Northern Hemisphere autumn equinox. We currently divide the year into four seasons based upon the solstices and equinoxes, with our summer ending on the equinox in September. This approximates summer as the quarter of the year with the warmest temperatures. Much of pre-Christian northern Europe celebrated "cross-quarter days" halfway between the solstices and equinoxes, and divided the seasons on these days. Using this definition, summer was the quarter of the year with the longest daily periods of daylight, with summer ending with Lughnasadh, traditionally celebrated on Aug. 1.

The planet Mercury will be passing on the far side of the Sun as seen from the Earth, called superior conjunction. Because Mercury orbits inside of the orbit of Earth, Mercury will be shifting from the morning sky to the evening sky and will begin emerging from the glow of the dusk on the western horizon on or after Aug. 9 (depending upon viewing conditions).

August 2

Early Monday morning, Aug. 2, 2021, the planet Saturn will appear opposite the Sun as seen from the Earth (called "opposition"). Saturn will be at its closest and brightest for the year, effectively a "full Saturn," rising around sunset and setting around sunrise. With clear skies and a backyard telescope, you should be able to see the rings of Saturn and its largest moon, Titan.

At 3:36 a.m. EDT on Monday, the Moon will be at apogee, its farthest from the Earth for this orbit.

August 3

On Tuesday morning, Aug. 3, 2021, the bright star Aldebaran will appear about 6 degrees to the right of the waning crescent Moon.

August 6

On Friday morning, Aug. 6, 2021, the bright star Pollux will appear to the lower left of the thin waning crescent Moon.

August 8

Sunday morning, Aug. 8, 2021, at 9:50 a.m. EDT, will be the new Moon, when the Moon passes between the Earth and the Sun and will not be visible from the Earth.

The day of – or the day after – the New Moon marks the start of the new month for most lunisolar calendars. The seventh month of the Chinese calendar starts on Sunday, Aug. 8, 2021 (at midnight in China's time zone, which is 12 hours ahead of EDT). Sundown marks the start of Elul in the Hebrew calendar. Elul is a time of preparation for the High Holy Days of Rosh Hashanah and Yom Kippur. Customs include granting and asking others for forgiveness as well as beginning or ending all letters with the wish that the recipient will have a good year.

In the Islamic calendar, the months traditionally start with the first sighting of the waxing crescent Moon. Many Muslim communities now follow the Umm al-Qura Calendar of Saudi Arabia, which uses astronomical calculations to start months in a more predictable way. Using this calendar, sundown on Aug. 8, 2021, will probably mark the beginning of Muharram. Muharram is the first month of the Islamic year and one of the four sacred months during which warfare is forbidden.

August 9

Monday evening, Aug. 9, 2021, will be the first evening that the planet Mercury will appear above the horizon in the west-northwest about 30 minutes after sunset (an approximation of when it will begin being visible in the glow of dusk). For this apparition, Mercury will remain low and not appear above the horizon as evening twilight ends.

Before evening twilight ends on Monday, you might be able to see the waxing crescent Moon low in the west-northwest with the planet Mars to the lower left of the Moon. Mars will set first about 9 minutes before evening twilight ends at 9:05 p.m. EDT, evening twilight will end at 9:14, and the Moon will set about 3 minutes later at 9:17 p.m. Mars will be nearing the opposite side of the Sun from us, appearing near its farthest and faintest, so it may be difficult to see in the glow of dusk without binoculars or a telescope.

August 10

Tuesday evening, Aug. 10, 2021, the bright planet Venus will appear in the west to the left of the waxing crescent Moon. They will be about 5 degrees above the horizon as evening twilight ends at 5:12 p.m. EDT, Venus will set first at 9:42 p.m., and the Moon will set about 4 minutes after Venus.

By Wednesday evening, Aug. 11, 2021, the Moon will have shifted such that the bright planet Venus will appear to the lower right of the waxing crescent Moon. Venus will set first at 9:41 p.m. EDT.

August 12

The Perseid meteor shower is expected to peak on (or after) Thursday afternoon, Aug. 12, 2021 (when we can't see these meteors). Since this shower tends to have a broad peak you may be able to see meteors in either the early morning before or after this expected peak. The night before the expected peak, if the skies are clear and you are in a dark place, try looking for meteors between moonset (10:13 p.m. EDT) Wednesday night, Aug. 11, and the first signs of dawn (about 4:39 a.m.) Thursday morning, Aug. 12.

The night after the expected peak of the Perseid meteor shower, try looking for meteors between moonset (10:40 p.m. EDT) Thursday night, Aug. 12, and the first signs of dawn (about 4:40 a.m.) Friday morning, Aug. 13, 2021.

August 13

Friday evening, Aug. 13, 2021, the bright star Spica will appear to the lower right of the waxing crescent Moon.

August 14

Saturday morning, Aug. 14, 2021, will be the last morning that the planet Saturn will appear above the horizon at the time morning twilight begins.

Sometime during the morning to early afternoon on Aug. 14 (2021-Aug-14 14:35 UTC with 4 hours, 10 minutes uncertainty), Near-Earth Object (2016 BQ), between 38 to 85 feet (12 and 26 meters) across, will pass the Earth at between 4.3 and 4.6 lunar distances (nominally 4.4), traveling at 10,500 miles per hour (4.70 kilometers per second).

August 15

On Sunday, Aug. 15, 2021, the Moon will appear half-full as it reaches its first quarter at 11:20 a.m. EDT.

August 16

Monday evening, Aug. 16, until Antares sets just after midnight Tuesday morning, Aug. 17, 2021, the bright star Antares will appear to the lower right of the waxing gibbous Moon.

August 17

Tuesday morning, Aug. 17, 2021, at 5:16 a.m. EDT, the Moon will be at perigee, its closest to the Earth for this orbit.

August 18

Before evening twilight ends on Wednesday, Aug. 18, 2021, you might be able to see the planets Mercury and Mars appearing only 0.14 degrees (or about a quarter of the apparent diameter of the Moon) apart from each other. To see them you will need a clear view to the horizon low and slightly to the north (right) of due west. They will only be about 2.5 degrees above the horizon about 30 minutes after sunset (8:28 p.m. EDT) and Mercury will set first about 15 minutes later (8:43 p.m.), with evening twilight ending 17 minutes after that (9 p.m.). Mercury will appear substantially brighter than Mars (which may be hard to see in the glow of dusk). In addition, the bright planet Venus will appear about 20 degrees to the upper left of Mercury and Mars.

August 19

Thursday afternoon, Aug. 19, 2021, the planet Jupiter will appear opposite the Sun as seen from the Earth (called "opposition"). Jupiter will be at its closest and brightest for the year, effectively a "full Jupiter," rising around sunset and setting around sunrise. With clear skies and a backyard telescope, you should be able to see Jupiter's four bright moons, Ganymede, Callisto, Europa, and Io, shifting positions noticeably over the course of an evening.

August 20 to 21

Friday evening into Saturday morning, Aug. 20 to 21, 2021, the planet Saturn will appear near the waxing full Moon. Saturn will appear about 5 degrees above the Moon as evening twilight ends and will appear to move clockwise around the Moon until Saturn sets in the west-southwest Saturday morning (at 4:54 a.m. EDT).

Saturday evening into Sunday morning, the bright planet Jupiter will appear near the full Moon. Jupiter will appear about 6 degrees to the upper left of the Moon as evening twilight ends and will appear to shift clockwise around the Moon as the night progresses. The Moon will reach its highest in the sky for the night early on Sunday morning at 1:04 a.m. EDT when Jupiter will appear above the Moon, and Jupiter will appear to the upper right of the Moon as morning twilight begins at 5:27 a.m.

The full Moon after next will be on Sunday morning, Aug. 22, 2021, at 8:02 a.m. EDT. The Moon will appear full for about 3 days around this time, from Friday night through Monday morning, making this a full Moon weekend. ​

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