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Two Stars With a Common Thread

by Geoff Chester, USNO Public Affairs | 18 July 2023

by Geoff Chester, USNO Public Affairs | 18 July 2023

Crescent Venus, imaged at the U.S. Naval Observatory, 2017 March 9, 18:50 UT
with the 30.5-cm (12-inch) f/15 Clark/Saegmüller refractor, an Antares 1.6X 2-inch Barlow lens
and a Canon EOS Rebel T2i DSLR.

The Moon returns to the evening sky this week, waxing through her crescent phases as she moves southward along the ecliptic through the late springtime constellations.  First Quarter occurs on the 25th at 6:07 pm Eastern Daylight Time.  Look for Luna in evening twilight near bright Venus on the evenings of the 19th and 20th.  Use binoculars to locate Mars a few degrees east of the Moon on the 20th.  On the evening of the 24th you will find the Moon just over two degrees from the bright star Spica.

At the end of evening twilight several bright stars begin to appear.  The brightest of these is Arcturus, which is high in the west at 10:00 pm.  Arcturus is the brightest star in the northern hemisphere of the sky and fifth-brightest overall.  It sports a pleasing rosy hue that tells astronomers that it is an evolved giant star that is gradually swelling up as it exhausts the hydrogen fuel in its core.  As its surface grows larger its surface temperature cools, giving it a more reddish hue.  It is one of the closest such “red giant” stars to us, located about 37 light-years away.  South of Arcturus is blue-tinted Spica, the brightest star in the constellation of Virgo.  Spica is actually a very close pair of blue giant stars that orbit each other in just four days.  The pair shine with a combined luminosity of some 21,000 Suns from a distance of some 250 light-years.  Looking due south, you will spot another red-hued luminary, Antares, the brightest star in the constellation of Scorpius.  Like Arcturus, Antares is an evolved star that is fusing hydrogen in a shell around its core.  Unlike Arcturus, though, Antares is much more massive and is nearing the end of its life cycle.  It has swollen to a vast size that would engulf the orbit of Mars if it was located in our solar system, and its low surface temperature gives it its ruddy hue, leading to its name, which translates to “Rival of Mars”.  It lies some 550 light-years away.

Antares shares an interesting mythological link with a similar star that graces the mid-winter nights.  Betelgeuse, one of the two brightest stars in Orion, is another example of a massive red supergiant star.  Located at about the same distance as Antares, the two stars are the brightest examples of their kind in the sky, but you will never see them in the sky at the same time.  In Greek mythology, Orion was a half-mortal son of Poseidon gifted with great strength and hunting prowess.  At one point he declared before Gaia, the goddess of the Earth, that he would kill all of the animals on the planet.  Gaia was, of course, outraged, and sent a lowly scorpion to dispatch the boastful hunter.  The scorpion succeeded, but Orion was saved by the healing powers of Ophiuchus.  In order to maintain harmony in the heavens, Zeus placed Orion and Scorpius in the sky, but arranged them to be opposite each other so they would never encounter each other again.  Zeus also placed Ophiuchus in the sky directly above Scorpius to ensure that the beast would stay in its assigned celestial spot.

This is probably your last week to get a good look at the bright glimmer of Venus.  The dazzling planet is plunging rapidly toward the western horizon, setting five minutes earlier on each successive evening.  You should be able to spot her in twilight near the slender crescent Moon on the evenings of the 19th and 20th.  

Mars may be found just over three degrees to the left of the Moon on the evening of the 20th.  You might have difficulty seeing him in twilight half an hour after sunset, so use binoculars to find his pale red glow.  The red planet is also losing ground to the advancing Sun, but he only sets two minutes earlier each evening.  

Saturn should be an easy object to spot in the southeastern sky by local midnight.  The ringed planet is slowly drifting westward against the faint stars of the constellation Aquarius, one of the most obscure star patterns in the sky.  Saturn shines with a pleasing yellow hue, and a peek through a small telescope will confirm his identity.  The planet’s distinctive rings are tipped about 10 degrees to our line of sight, and should be easy to see at low power magnifications.

Jupiter now rises at around 1:00 am local time and is high in the east as morning twilight begins.  Old Jove will be the brightest object in the early morning sky and will be a great target for early morning telescopic viewing.  The hours before sunrise offer the steadiest air during these hot summer days, so you might have your best views of the giant planet during these hours.


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

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