by Geoff Chester, USNO Public Affairs | 01 May 2024 Vega Rising over Desolation Canyon, Utah, 2018 June 20 The Moon begins to climb northward along the ecliptic this week, waning through her crescent phases as she joins the rising stars of autumn in the pre-dawn sky. Look for Luna near the yellow glimmer of Saturn on the mornings of May 3rd and 4th. New Moon occurs on the 7th at 11:32 pm Eastern Daylight Time. May 1st is May Day, an ancient calendar marker called a “cross quarter” day that marked the mid-point of an astronomical season. These dates, along with the solstices and equinoxes, were traditional days in Medieval times when serfs paid their rent to their feudal masters, usually in the form of livestock or grain. Each season has a cross-quarter day, and although their observance has mostly been lost to history, we still unwittingly celebrate several of them. The three that most of us are familiar with are Halloween, Groundhog Day, and May Day. Lammas, which falls on August 1st, is the least observed today. Cross quarter days are still preserved in some traditions around the globe, especially in Celtic cultures, as the beginnings of seasons. This is why you will find that many people refer to June 21st as “Midsummer’s Day”. The current Globe at Night citizen science observing campaign is underway and runs through the duration of the week. This month’s featured constellation is Leo, the Lion, one of the signature constellations of spring. Leo’s brightest star, Regulus, may be found high in the south near the meridian as evening twilight fades. Regulus is the 21st brightest star in the sky and is easily seen from urban locations. The rest of Leo requires a good transparent night to find in city-bound skies, but should be easy to spot from suburban locations. Leo consists of two distinct asterisms. The first is his “mane” formed by a semicircle of stars to the north of Regulus, known as the Sickle. The brightest star in the Sickle as a beautiful gold-hued double star, Algieba, whose components can be split in a good 3-inch telescope. Some 20 degrees east of Regulus you should be able to spot a right triangle of stars, with the second-magnitude star Denebola indicating the tip of Leo’s “tail”. Between the Sickle and the triangle are a number of third- and fourth-magnitude stars that flesh out the constellation, and it doesn’t take a large stretch of imagination to see the crouching figure of the regal cat. Use the Globe at Night web app to find comparison star charts and to report your findings. As Leo wends westward during the night, the eastern sky is dominated by the bright star Arcturus, the brightest star in the northern sky. By midnight Arcturus approaches the meridian, and if you look to the northeast you will see another bright star on the rise. This is Vega, brightest star in the Summer Triangle asterism. Vega is another relatively nearby star, just 25 light years away. Unlike Arcturus, which has evolved to its red giant phase, Vega is a star in its “prime”, having formed some only some 500 million years ago. It has been extensively studied by astronomers. Once considered to be a standard “calibration” star, we now know that it is far from “ordinary”. It is surrounded by a dusty disc and harbors at least two planets, and it is one of the fastest rotating stars known. Where the Sun takes a leisurely 27 days to rotate once, Vega spins once every 16.5 hours! Jupiter is now falling rapidly toward the Sun. You may still be able to glimpse the giant planet in bright twilight as May begins, but Old Jove will pass behind the Sun on the 18th. We won’t see him back in the evening sky until late summer. Saturn is now visible in the pre-dawn sky, rising at around 4:00 am. You will find the ringed planet about 10 degrees above the brightening southeast horizon at around 5:00 am.