Sky Watch

by J. Alex Knoll

Standing Up to the Moon

July’s Thunder Moon deadens all but the brightest lights

The gibbous moon waxes through southern skies this week, becoming full on the 25th. July’s full moon is known as the Hay Moon or the Thunder Moon. Rising at dusk and setting at dawn, the full moon dominates the sky this week, blotting out all but the brightest planets and stars.

As the sun sets in the northwest before 8:25 this week, the first light to appear is the evening star Venus, 20 degrees above the western horizon. As dusk gives way to darkness, Venus is joined by ruddy Mars and golden Saturn to the left. This week Venus races westward night by night, leading up to a three-planet conjunction next week.

To the right of Venus, hugging the western horizon at nightfall, is bluish Regulus, the brightest star in the constellation Leo. Closer still to the horizon is our solar system’s innermost planet, Mercury, shining twice as bright as Regulus. At magnitude –0.2, Mercury is outshined by only Venus, Jupiter and Canis Major. Yet because this elusive planet never strays farther than 20 degrees from the sun’s blinding glare, it can be one of the most difficult objects to see. Over the coming nights Mercury inches higher, closing the gap between Regulus until Tuesday, when they appear less than one-half degree apart.

Jupiter rises in the east after 11pm and dominates the sky between the dim constellations Aquarius and Pisces. By the approach of daybreak, around 6am this week, Jupiter is high in the south.

Between dusk and dawn, summer’s bright stars stand up to the glare of the moon. Look for rosy-hued Arcturus, the fourth-brightest star, high in the west at sunset. Below it is Spica, the 16th brightest star, and low in the south shines Antares, the 19th brightest. Far to the east are the stars of the Summer Triangle: Vega, the fifth-brightest star; Altair, the 12th-brightest; and Deneb, the 19th-brightest.