SKY GUIDE: This chart represents the sky as it appears over Maine during July. The stars are shown as they appear at 10:30 p.m. early in the month, at 9:30 p.m. at midmonth and at 8:30 p.m. at month’s end. Saturn and Jupiter are shown in their midmonth positions. To use the map, hold it vertically and turn it so that the direction you are facing is at the bottom. Sky chart prepared by George Ayers.
The month of July is named after Julius Caesar. This is the first full month of summer and the days are already getting several minutes shorter, but they are still well over 15 hours long. There are many interesting highlights for everyone to observe in both the morning and evening skies during this warm summer month.
Two planets, a dwarf planet and an asteroid will reach opposition this month. Three planets will grace our evening sky again after a long absence of evening planets and two planets will make dramatic appearances in our morning sky. There will also be some nice conjunctions of the moon with Jupiter and Saturn in the evening sky and Venus and Mercury in the morning sky. Then the first good meteor shower since the April Lyrids will be the Delta Aquarids. which will peak on July 28. Then there will even be another partial penumbral lunar eclipse.
Jupiter is rising about 4 minutes earlier each evening. The king of the planets starts this month rising at about 9:30 p.m. and it will rise right at sunset on July 14 when it reaches opposition. Jupiter went into retrograde – or westward motion – against the fixed background of stars two months ago and it will end its retrograde in two more months.
When a superior planet reaches opposition, it is always right in the middle of the retrograde loop that it traces through our sky. This is only an optical illusion since none of the planets actually stop moving forward and then back up for a few months. It only seems that way because we are orbiting in the same plane around the sun, which is called the ecliptic. As the faster planet, earth, in this case, catches up with and passes the slower moving planet, Jupiter, the slower planet seems to be moving backwards in our sky. This happens to all of the superior planets from Mars out to Neptune as we catch up with and pass them in our faster, tighter orbit around the sun. There were many clever and involved explanations for this when we still believed we lived in a geocentric solar system, but once we figured out that we actually live in a heliocentric solar system, all of this motion could be explained very simply.
Saturn rises shortly after Jupiter and will reach its own opposition about a week later, on July 20. The ringed planet is twice as far away as Jupiter. At nearly 1 billion miles away, it takes light 1 hour and 20 minutes to get to us from Saturn. Notice that Saturn has a slight golden hue and is fully 15 times, or three magnitudes, fainter than Jupiter. You will be able to see Titan and a few of the other larger moons of Saturn in a telescope. You can easily see all four of the larger Galilean moons of Jupiter through a telescope. Jupiter now has 79 known moons and Saturn is up to 82.
Mars starts the month rising just after midnight and it will end the month rising by 11:15 p.m. The red planet gets dramatically brighter and larger this month as we catch up with it in our orbits around the sun, even though it will not reach its opposition until the middle of October.
Brilliant Venus reappeared in our morning sky last month and will make a dramatic climb higher into our morning sky this month. This will be its steepest climb in eight years, which is the length of time a cycle of Venus orbits takes. Venus will pass through the Hyades star cluster in Taurus this month and the waning crescent moon will be visible close to it on July 17 and then it will pass near Mercury low on the horizon on July 19 half an hour before sunrise.
The Delta Aquarids will peak on the morning of July 28. They start more than a week earlier and will overlap with the Perseids, which will not peak until the Aug. 12, but start several weeks before. The Delta Aquarids are caused by Comet 96/P Machholz and you will see tiny, sand grain-sized pieces of this not-so-famous comet smash into our atmosphere at 25 miles per second, which is just a little faster than we orbit the sun. The moon will be just past first quarter, so it will interfere a little until it sets around 1 a.m.
The third largest asteroid, Pallas at 317 miles across, will reach opposition in Vulpecula the Fox on the 12th. It will only reach 9.6 magnitude, so would need a good pair of binoculars or a telescope to see it for yourself. The dwarf planet Pluto will reach opposition in Sagittarius on the 15th. It spends 20 years in each of the 12 zodiac constellations on its 248-year orbit, so it never appears to move much in our sky.
JULY HIGHLIGHTS
July 4: Earth is at aphelion, or farthest from the sun today at just over 94.5 million miles, or about 3% farther away than it is at perihelion on Jan. 4.
July 5: Full moon is at 12:44 a.m. This is also called the Hay or Thunder Moon. Isaac Newton published his famous Principia on this day in 1687. The moon passes near Jupiter and Saturn tonight.
July 11: Mars and the moon are just six degrees apart this morning and Venus and Aldebaran in Taurus are just one degree apart.
July 12: Last quarter moon is at 7:30 p.m.
July 14: Jupiter is at opposition. New Horizons passed close to Pluto on this day in 2015.
July 15: Jocelyn Bell Burnell was born on this day in 1943. She discovered the first pulsar, which is a rapidly spinning neutron star, the result of a supernova explosion like the Crab Nebula, in 1967 with a radio telescope.
July 17: The moon is near Venus this morning and near Mercury on the 19th.
July 20: Saturn is at opposition all night. New moon is at 1:34 p.m.
July 23: Vera Rubin was born on this day in 1928. She studied stellar rotations in galaxies and first proposed dark matter as being the cause of all the stars moving at about the same speed.
July 27: First quarter moon is at 8:34 a.m.
July 28: The moon is near Antares in Scorpius this evening.
Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England.
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