The month of July is named for Julius Caesar. On this first full month of summer, the days are still long and the nights are still short, but it is a great time to get outside under our warm skies to see and appreciate just a few of its myriad wonders.

All eight of the planets in our solar system will be visible at some time this month in our evening sky. We are standing on one of them. Our home planet will reach aphelion, or its greatest distance from the sun, on Monday, July 4. We will be 94,512,904 miles away for the sun, or only about 3 percent farther than we are at perihelion in early January. There will also be a new moon a few hours earlier on that same day.

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, Mars and Jupiter are shown in their midmonth positions. To use the map, hold it vertically and turn it so the direction you are facing is at the bottom. Sky chart prepared by George Ayers

Jupiter is still in the eastern part of Leo, moving in its normal, eastward direction along the ecliptic. The king of the planets is slowly fading as we pull farther ahead in our faster orbit around the sun, but it is still brighter than anything except Venus, the moon and the sun. By the end of the month, Jupiter will set less than two hours after sunset. Back on March 8, when it was at opposition, Jupiter was rising just as the sun was setting.

A spacecraft named Juno will arrive at Jupiter on July 4. Launched on Aug. 5, 2011, by a powerful Atlas 5 rocket, Juno is one of the 10 missions that have studied Jupiter at close range. Juno is the first spacecraft sent to the outer solar system that is not powered by the radioactive decay of plutonium. Instead, Juno has three huge solar-cell panels, each of which is nearly 30 feet long. They only generate 400 watts of power, as Jupiter is about five times farther away at 500 million miles, or about 45 minutes at the speed of light.

The other nine spacecraft have taught us many important things about this great planet, but we have only scratched the surface of what this planet can tell us. Juno will drop into a sequence of 14-day-long orbits by October, which will carry it to 2,600 miles above its cloud tops as it plunges in over the North Pole and exits again over the South Pole and then out to 1.6 million miles away. This avoids most of the trapped charged particles that could easily destroy this spacecraft.

Juno will be racing along at 37 miles per second, or twice as fast as we are always orbiting the sun. Just picture this amazing little spacecraft with its triangle of giant solar panels and nine incredible scientific instruments slowly rotating in a cartwheel every 30 seconds while it is gathering valuable data about our largest planet. Juno will be racing through incredibly strong magnetic fields and millions of amps of electric current. It is expected to make about 35 such dramatic and dangerous orbits, but it could last much longer.

We don’t even know if Jupiter has a solid core of metal, rock and ice, or essentially no core at all beyond its metallic hydrogen. The pressures at the center of Jupiter are 50 million times greater than what we experience on the surface of the earth. When NASA dropped the Galileo probe down to 100 miles below Jupiter’s cloud tops 21 years ago, it found the expected ammonia layers but not the water layer. So Jupiter may have far less water than expected, which would have major implications for how it was formed and for how most other Jupiter-sized planets in other solar systems have formed.

Jupiter has very intense northern and southern lights around its poles, generated by its powerful dynamo of rapidly spinning metallic hydrogen. Jupiter is 10 times larger than the earth and 318 times heavier, but it rotates one full turn every 10 hours. This huge magnetosphere extends about 3 million miles into space toward the sun, but its giant geomagnetic tail extends all the way past Saturn, half a billion miles away, on the other side of the sun. Our earth also has a magnetosphere, stretching roughly to the moon on the side away from the sun, but it is more than 1,000 times smaller.

Each of Jupiter’s four large Galilean moons, visible with a good pair of binoculars, leaves a bright knot of light as a footprint in these polar auroras. The one left by Io, the most volcanic place in our solar system, is especially interesting because about a ton of sulfur dioxide escapes from its thin atmosphere every second, forming a huge doughnut-shaped torus of plasma that interacts with Jupiter’s magnetic fields, generating three distinctly audible radio sounds.

Look for a slender waxing crescent moon passing just below Regulus in Leo and then Jupiter about one hour after sunset on the 7th and 8th – just a few days after Juno will get to Jupiter.

Then keep watching as the moon gets about seven percent larger and travels 12 degrees farther east each night. The waxing gibbous moon will be right above Mars on the 14th, then right above Saturn the next evening. Both brilliant orange Mars and golden Saturn are now slowly fading like giant cooling embers, but they are still brighter and closer than usual. Also look for the orange giant star named Antares in Scorpius below this ever-changing trio of bright celestial objects. At 700 times the size of our sun, Antares is one of the largest stars in the Milky Way galaxy.

Both Venus and Mercury reappear during the final week of July, about five degrees apart and very low in the western sky, setting about 45 minutes after sunset. Look for them near Regulus in Leo, not far from Jupiter. Neptune is now in Aquarius, and Uranus is in the next constellation to the east, Pisces, rising in the late evening.

Pluto is at opposition on July 7 and will spend the whole season in the Teaspoon asterism. Pluto takes 248 years to orbit the sun, so it will not appear to move much in our sky.

Several meteor showers will happen later this month after a long lull in activity. These include the Delta Aquarids, the Piscis Austrinids and the Alpha Capricornids. You will also see some early Perseids meteors, which will peak by Aug. 12.

JULY HIGHLIGHTS

July 4: Earth is at aphelion, or farthest from the sun. Juno will arrive at Jupiter. The Crab nebula in Taurus was first seen in 1054 by many cultures around the earth. This supernova is about 6,500 light years away, which means that this giant star actually exploded about 7,500 years ago, but its light just reached us about 1,000 years ago. New moon is at 7:01 a.m.

July 5: In 1687, Isaac Newton published his Principia, explaining his Laws of Universal Gravitation and many other principles of math and physics.

July 8: The moon is near Jupiter this evening.

July 11: First-quarter moon is at 8:52 p.m.

July 15: The waxing gibbous moon, Saturn and Antares form a nearly vertical line in the south about one hour after sunset.

July 16: In 1994, the first of 21 fragments of Comet Shoemaker-Levy 9 hit Jupiter. Another piece hit the planet about every six hours.

July 19: Full moon is at 6:56 p.m. This is also called the Hay or Thunder Moon.

July 20: In 1969, Neil Armstrong and Buzz Aldrin set foot on the moon. Only 10 more humans have walked on the moon, none since December 1972.

July 26: Last-quarter moon is at 7 p.m.

July 29: The moon will occult Aldebaran in Taurus just after sunrise.

Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England

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