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Thunder Moons and Summer Nights

by Geoff Chester, USNO Public Affairs | 27 June 2023

by Geoff Chester, USNO Public Affairs | 27 June 2023

Almost a Thunder Moon, imaged 2010 June 21 at the U.S. Naval Observatory
with a Canon EOS Rebel T2i DSLR

The Moon waxes in the evening sky, moving southward along the ecliptic as she brightens to the very aptly named full Thunder Moon, which occurs on July 3rd at 7:39 am Eastern Daylight Time.  As with each month’s full Moon, there are number of other names based on the traditions and folklore of different cultures.  July’s full Moon is also widely known as the Buck Moon, since the new year’s growth of antlers begins to appear on male deer.  It is also known as the Hay Moon, Raspberry Moon, or Salmon Moon.  This year the almost full Moon might also be called the “Fireworks Moon”, since it will offer some cosmic competition to our traditional Independence Day evening activities.  Look for the Moon just to the northeast of the bright star Spica on the evening of June 27th.  On the 30th she will be just three degrees west of the ruddy star Antares.

As June ends, we see the latest sunsets of the year beginning to recede to earlier times.  For most of the week sunset will occur at 8:38 pm EDT here in Washington, but by July 4th he will set one minute earlier.  By the end of July sunset will occur at 8:21 pm, and as July turns to August sunset will average just over one minute sooner each evening.  You still have plenty of daylight to enjoy outdoor activities in the evening, but if you are an amateur astronomer it’s nice to know that evening twilight will soon end before your bed time! 

As the Moon waxes this week we are seeing the return of the bright stars of summer.  High in the east at around 10:30 pm you will see a trio of blue-white beacons tracing out a large triangle in the sky.  They form an asterism called, naturally, the Summer Triangle, and each star has a story to tell.  

The highest and brightest is Vega, which leads the diminutive constellation of Lyra, the Harp.  Vega is the fifth-brightest star in the sky and has a distinctive blue tint.  Its brightness is due to its intrinsic brightness and its relative proximity to the solar system.  It is a “main sequence” star that is in the middle of its evolution.  With just over twice the mass of the Sun, it fuses hydrogen at a faster rate and hotter temperature, some 40 times the luminosity of Old Sol.  At a distance of 25 light years, it is one of the most studied stars in the sky.  Thanks to the slow precession wobble of the Earth’s rotational pole, Vega will be our “pole star” in about 12,000 years.

The southernmost star in the Triangle is Altair, which leads Aquila, the Eagle.  Altair is similar to Vega in physical characteristics, but it is half as massive and about a quarter as luminous.  Located just 16.7 light years from us, it is one of the closest bright stars to the solar system.  It is unusual in that it has an extremely fast rotation period of just 7.7 hours while the Sun spins at a much more leisurely 25ays!

The northernmost star, and the last to rise, is Deneb, which marks the “tail” of Cygnus, the Swan.  While it appears fainter than its companions, it is actually a stellar powerhouse.  Deneb lies over 100 times farther away than Vega, which means that it must have a luminosity of around 200,000 Suns!  Deneb has consumed all of the hydrogen in its core and is now fusing helium into heavier elements.  Within a few million years it will become a red supergiant star like Antares, currently visible in the southern part of the sky.  By contrast, our Sun still has a few billion years to go before it exhausts the hydrogen in its cor.

Venus is still a fixture in the evening twilight sky, but you may have noticed that she is beginning to set earlier each night.  The dazzling planet has “rounded the corner” on her faster, inner solar orbit and is now beginning to overtake Earth.  During July she will pull an almost Houdini-like vanishing act, and by the end of the month you will be hard-pressed to see her.  If you have a small telescope, watch her disc increase in size and decrease in phase as she nears solar conjunction.

Mars may be found a few degrees east of Venus this week, but he will keep plodding eastward as Venus takes her dive.  The red planet will steadily close the gap with the bright springtime star Regulus and will undergo a close conjunction with the star on July 9th and 10th.

Late night skywatchers can catch Saturn rising in the southeast shortly before midnight.  The ringed planet will make steady progress into the evening sky over the next couple of weeks.  His yellow hue will distinguish him, and he will be the brightest object in that part of the sky.

Early risers can spot Jupiter’s cheery glow in gathering morning twilight.  The giant planet hangs over the eastern horizon in the hour before dawn.  You should have no trouble spotting him until the Sun peeks over the skyline.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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