by Geoff Chester, USNO Public Affairs | 06 July 2023 Messier 13, the Great Hercules Cluster, imaged 2021 August 28 from Mollusk, Virginia with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor and a ZWO ASI183MC CMOS color imager. Note the distant external galaxy NGC6207 at upper left edge. The Moon wanes in the morning sky this week, turning northward along the ecliptic as she wends her way through autumn’s rising constellations. Last Quarter occurs on the 9th at 9:41 pm Eastern Daylight Time. Look for Luna southeast of Saturn during the early morning hours of the 7th. She ends the week near Jupiter in the gathering morning twilight. Earth reaches aphelion on the 6th at 4:06 pm EDT. At this time, we will reach the point in our orbit that is most distant from the Sun, at a distance of 152,095,415 kilometers (94,507,710 miles) from the Day Star. Six months from now we will pass perihelion, some 5 million kilometers (3.1 million miles) closer to Old Sol. This annual excursion means the Earth’s orbit deviates from a circle by about 1.6 percent, so the effects on our overall climate is minor. However, since aphelion is the time when Earth travels at its slowest speed in its annual orbital excursion, and it occurs near the summer solstice, boreal summer is our longest season, lasting some 93 days. Conversely, boreal winter is the shortest season, lasting just 89 days. You probably haven’t noticed, but the days are getting shorter. Most of the daily change is in the time of sunrise, though, with the Sun now rising about 8 minutes later than it did back in mid-June. Sunset occurs only a minute later than it did last week, but the net effect is giving us gradually longer nights. However, you still have to wait until after 10:30 pm local time for the sky to become fully dark at Washington's latitude. Skywatchers can take heart, though; by the end of July evening astronomical twilight ends about half an hour earlier! If you are willing to stay up until the late evening, take a bit of time to participate in the July observing campaign for the Globe at Night citizen-science program, which starts on the evening of the 8th. This month’s target constellation is Hercules, a sprawling constellation that is almost directly overhead at around 11:00 pm. To find it, look about one-third of the distance from the bright blue star Vega and the rose-tinted star Arcturus. Under a clear suburban sky you should be able to pick out a trapezoid of third-magnitude stars that form an asterism known as the “Keystone”. Chains of stars seem to radiate from each corner, outlining the rest of the constellation. Use the Globe at Night web app to match your view of Hercules with star charts to measure the darkness of your sky. The Keystone holds one of the northern sky’s best telescopic treats. Just below the star Eta Herculis, northwest corner of the Keystone, a pair of binoculars will show a round patch of light wedged between a pair of stars. If you point a telescope of four inches or more aperture at this hazy spot, it will begin to resolve into a tight cluster of innumerable faint stars. An 8-inch telescope will resolve the cluster to its core, and each increase in aperture will show more stars concentrated toward the object’s center. This object is Messier 13, the Great Hercules Cluster, the largest and brightest of the Milky Way’s globular star clusters visible from the northern hemisphere. Globular star clusters contain some of the oldest stars in the universe. Spectroscopic analysis show that they are composed mostly of primordial hydrogen and helium, and their ages date back to a time shortly after the Big Band. The origins of these clusters are still something of a mystery. Today it is thought that these are the remnant cores of dwarf galaxies that have passed through the plane of the Milky Way, stripping out the gas and dust that forms younger stars and only retaining the most ancient ones. There are over 100 such clusters associated with the Milky Way galaxy; about 80 are visible to northern hemisphere telescopes. Venus spends the week about 4 degrees to the west of Mars as the two objects gather around the bright star Regulus in the constellation of Leo, the Lion. The dazzling planet starts to make a slow turn toward the south as she begins her descent toward the Sun. She reaches her greatest brilliancy for the current evening apparition on the 7th. Mars slowly begins to widen the gap with Venus, and by the end of the week the red planet brushes by Regulus. The star and the planet will be closest on the evenings of the 9th and 10th, when they are less than one degree apart. They are best seen about an hour after sunset, when they will be about 15 degrees above the western horizon. Binoculars may help you to locate them in a hazy sky. Saturn now rises well before midnight, and you should be able to catch a glimpse of him in the southeastern sky during the early morning hours. You can see the ringed planet in the company of the waning gibbous Moon before dawn on the 7th. Jupiter is still best seen during the morning twilight hours. The giant planet should be easy to spot some 35 degrees above the eastern horizon at 5:00 am local time. Both Jupiter and Saturn will be in prime viewing position as we head toward late summer.