On July 12, the planet Neptune will have completed one orbit of the sun since its discovery by astronomer Johann Galle (Ger.) on Sept. 23, 1846. Although Galle spotted it by telescope, Neptune was actually discovered with pencil and paper. French mathematician Urbain LeVerrier predicted the existence of Neptune because of gravitational perturbations of the orbit of Uranus by an unknown planet. Galle, using LeVerrier’s calculations at the Berlin Observatory, sighted Neptune within one hour of starting his search. Neptune, the most distant planet in the solar system, is 30 astronomical units from the sun. The Earth is one AU from the sun where an AU is 93 million miles. The Neptune year is nearly 165 Earth years.
Focus on the Planets
Mercury is low in the west about 45 minutes after sunset. Around mid-month, Mercury fades and
drops from view. Venus rises about an hour before the sun as July opens very low in the east-northeast. Venus will vanish in the sun’s glare as the month draws to an end and not reappear until September.
Mars rises in the east but is dim and difficult to spot. On July 27, Mars teams up with a slim crescent moon. Jupiter rises about 2 a.m. as July opens and around midnight as the month closes. It remains up for the rest of the night and is prominent in the southeast at dawn. Jupiter
grows steadily larger and brighter and surface features such as the bands and the movements of its four moons are easily watched by telescope.
Saturn rises in the southwest about an hour after sunset. Saturn’s ring system has opened slightly but is still close to its maximum closure for the year. Viewers can console themselves with the moon Titan that orbits the planet every 16 days.
Uranus is a blue-green disc in the southeast predawn sky near Alpheratz in the Square of Pegasus. Neptune is a blue-gray disk high in the south after midnight.
8 Moon in first quarter, 1:03 a.m.
12 Neptune completes its first orbit since its discovery in 1846.
15 Full moon, 2:38 a.m. The full moon of July is called the Thunder Moon, Buck Moon, or the Hay Moon.
21 The moon is at apogee or farthest distance from Earth today. The sun enters Cancer on the ecliptic.
23 The sun enters the astrological sign of Leo but astronomically has just entered Cancer. Moon in last quarter, 1:03 a.m.
24 Jupiter joins with the moon in the east at 2 a.m.
27 Look for faint Mars just to the lower left of the crescent moon in the east during the early predawn hours. Aldebaran is far to the upper right of the moon.
30 New moon, 2:39 p.m. This is the peak night for the Delta Aquarid meteor shower. Ideal viewing conditions should produce between 15-20 meteors per hour.
31 Sunrise, 5:19 a.m., sunset, 8:03 p.m.
Correction: A previous version of this story misstated that Uranus would complete its first orbit since its discovery in 1846 on July 12, instead of Neptune.