By Matthew Johnson

Twilight is the time when the celestial sphere is partially illuminated. This occurs only before sunrise and after sunset.

There are three ranges of twilight, defined according to the sun’s position below the horizon. Civil twilight occurs when the sun’s upper limb is below the horizon and its center 6 degrees or less below the horizon. Nautical twilight occurs when the center of the sun is between 6 and 12 degrees below the horizon. Astronomical twilight occurs when the sun’s center is between 12 and 18 degrees below the horizon.

Astronomical dusk occurs at 18 degrees, beyond which the sky’s illumination by the sun is imperceptible. The best time to observe the heavens is after astronomical dusk. 

Summer starts on Saturday, June 20, at 5:44 p.m. The summer solstice occurs when the sun reaches its greatest declination, 23.5 degrees north of the celestial equator (the great circle on the celestial sphere directly above the terrestrial equator). On the day of the summer solstice, we have the most daylight and the shortest night of the year. The sun will rise on the East Coast at 5:07 a.m. and set at 8:23 p.m. to give us 15 hours and 18 minutes of daylight.

Many years ago I visited Stonehenge on the Salisbury Plain in England, a pilgrimage for an astronomer. On the summer solstice, the sun rises directly behind the Heel Stone at the ancient entrance to the stone circle. The sunlight is then channeled into the center of the monument. Solstices have been celebrated at Stonehenge for thousands of years. 

Throughout June, look for Jupiter close to Saturn low in the southern sky as dawn breaks. Saturn is found just east, to the left of brilliant Jupiter. There is a wonderful conjunction of the Moon and Venus in the morning twilight on June 19. It is very low in the sky and binoculars will help. Recall that with binoculars one can see Jupiter’s four largest moons.

A recently discovered comet made its closest pass to the earth in May at a distance of 52 million miles—a little more than half the distance from Earth to the Sun. The comet, known as Comet SWAN but officially named C/2020 F8 (SWAN), is now visible to the naked eye in the southern hemisphere. See the NASA website: https://apod.nasa.gov/apod/ap200514.html

The full moon this month, known as the Strawberry Moon, is on June 5. The new moon will appear on June 21.