by Geoff Chester, USNO Public Affairs | 14 May 2024 Moon and Thunderhead, imaged at the U.S. Naval Observatory, June 2010 The Moon wends her way through the spring constellations this week, waxing through her gibbous phases as she moves southward along the ecliptic. Last Quarter occurs on the 15th at 7:48 am Eastern Daylight Time. Look for Luna near the bright star Regulus in Leo on the evening of the 15th. Luna’s growing phase brightens the evening skies, hampering the view of the fainter stars of the season. One star that doesn’t mind Luna’s intrusion is Arcturus, the brightest star in the northern hemisphere of the sky. The star’s name derives from the ancient Greek, and loosely translates as “Guardian of the Bear”. While it is very easy to find, it helps to find other sights in the springtime sky. We can follow the arc traced out by the stars the form the “handle” of the Big Dipper as we “arc to Arcturus”, and if we continue that sight line we “speed on to Spica” the brightest star in the sprawling constellation of Virgo. Arcturus is a red giant star, entering the final stages of its lifetime. At a distance of 36.7 light years, it is the closest red giant star to the solar system. It has a notable rose tint, and it has always been a cheery sight to me as the cold nights of winter change to milder ones. It is the only star, apart from the Sun that I have seen in broad daylight, although that sighting in 1990 was made from the summit of Mauna Kea in Hawai’i. The star, known as H?k?le?a, the “Star of Joy” in Polynesian, has an intimate tie with the native Hawai’ians since it culminates directly over the islands. This allowed the ancient Polynesian navigators to find them as their culture spread across the Pacific Ocean. Sailing north and east from Tahiti, they would sail to the latitude where Arcturus was directly overhead, then turn westward to land on Hawai’i. Speeding on to Spica, we encounter a star that has a blue tint that is quite different from Arcturus. It is actually two stars that orbit each other once every four days; the components are so close to each other that they are elliptical in shape. The pair are located about 250 light years from us, and their combined luminosity is over 20,000 times that of the Sun. As midnight approaches, we can see one of the signature asterisms of summer climbing in the eastern sky. Vega, Deneb, and Altair form what is known as the Summer Triangle, and they will dominate the overnight hours as high summer approaches. Vega is the first to rise, its blue glow contrasting nicely with the rose of Arcturus. Vega is only 25 light years away and leads the diminutive constellation of Lyra, the harp. Altair, the southernmost apex of the triangle, is another nearby star at only 16 light years’ distance. It lies due east, about 10 degrees above the horizon at midnight. Deneb, the northernmost star in the triangle, may be spotted in the northeast. While it outwardly resembles its companions, it is actually one of the most luminous stars in the galaxy. It is estimated to be over 2000 light years from us, which means that it must shine with the equivalent light of some 200,00 Suns to appear as bright as it does in our sky. Early risers can spot the yellow glimmer of Saturn in the pre-dawn sky. You will find his yellow glow about 20 degrees above the southeast horizon at around 5:00 am. As morning twilight begins to brighten the sky, look for the ruddy glow of Mars low in the east. The red planet will gradually wend his way into the evening sky over the course of the year, dominating the sky as the year ends.