The Sky This Week
U.S. Naval Observatory's Weekly Blog

The Short Nights of High Summer

by Geoff Chester, USNO Public Affairs
14 June 2022

Summer Full Moon Rising, imaged from Alexandria, Virginia.
HDR composite image made with an 80mm (3-inch) f/6 Antares Sentinel refractor
and a Canon EOS Rebel SL2 DSLR camera.

The Moon starts the week scudding along the southern horizon, waning in the late night and early morning skies from her full phase to last quarter, which will occur on the 20th at 11:11 pm Eastern Daylight Time.  As mentioned last week, June’s Full Moon is known as the Strawberry Moon due to the warmer tone of her light shining through more of our atmosphere for Northern Hemisphere observers.  Early risers will find her climbing through the rising constellations of the autumnal sky.  She will be near the yellow glow of Saturn during the wee hours of the 18th and 19th, then cozies up to bright Jupiter before dawn on the 21st.

The summer solstice falls on the 21st at 5:14 am EDT.  This is the moment when the Sun reaches an ecliptic longitude of 90 degrees and appears to stop her northward motion along the ecliptic.  At this instant Old Sol will be directly overhead near the eastern shore of the Red Sea on the Arabian Peninsula.  The 21st is also the longest day of the year for residents of the Northern Hemisphere.  At the latitude of Washington DC Old Sol is above the horizon for 14 hours 54 minutes.  By comparison, Americans who live along the western states’ border with Canada will see 16 hours 8 minutes of daylight, while Key West sees just 13 hours 40 minutes.  One peculiarity of the solstices, though, is the difference between earliest sunrise and latest sunset.  This week we see the earliest sunrise of the year as the week begins, but the year’s latest sunset won’t occur until the 28th.  

As the week opens, the bright glow of the Moon washes out all but the night’s brightest stars.  The brightest of these, Arcturus, may be found high overhead as evening twilight fades.  Arcturus is the brightest star in the northern half of the celestial sphere and is the fourth-brightest star in the entire sky.  It has a cheery warm tint that has been described a “rose-hued” or “topaz”.  This color betrays its nature as an evolved star that has exhausted fusible hydrogen in its core and is entering the end stages of its evolution.  Hydrogen is now fusing into helium in a shell surrounding a helium core, causing the star to swell up and its surface to cool.  Although its mass is only slightly greater than that of the Sun, it shines with the luminosity of some 170 Suns.  At a distance of just 36.7 light-years from us, it is a natural beacon in our part of the Galaxy.

By midnight the three bright stars of the Summer Triangle asterism is climbing high above the eastern horizon.  The highest of these is the blue-hued star Vega, which leads the diminutive constellation of Lyra, the Harp.  Vega is only slightly dimmer than Arcturus, and it is firmly notched as the sky’s fifth-brightest star.  It is one of the closer bright stars to us at a distance of 25 light-years.  It was the first star to be photographed through a telescope in 1850.  The southernmost and second-brightest star in the Triangle is Altair, which lies a mere 16.6 light-years from us.  It is just under twice the mass of the Sun and shines with a luminosity of about 10 suns.  It has one of the fastest rotation periods of any star, spinning once on its axis in a mere 8.9 hours!  The final star in the Triangle is Deneb, which marks the “tail” of Cygnus, the Swan.  Although it appears to be almost as bright as its Triangle companions, it is actually one of the most intrinsically bright stars known in the sky.  It lies over 2,500 light-years away, so to appear as bright as it does in our sky it must be over 175,000 times the luminosity of Old Sol.

This week Saturn becomes the first bright planet to enter the evening sky this year.  The ringed planet rises at 11:58 pm EDT on the night of the 17th, and then rises four minutes earlier in each successive night.  You’ll still need to be up in the early morning to view him through the telescope, but it won’t be too much longer before you can enjoy a view of him at a decent hour.

Jupiter rises about two hours later, followed half an hour later by the brightening ruddy glow of Mars.  Venus joins the parade just before 4:00 am local time.  All of these planets will be visible in gathering twilight, and if you have binoculars look for Mercury between dazzling Venus and the horizon half an hour before sunrise.
 
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