The month of July was named for Julius Caesar and was once the fifth month of the year.
July is the first full month of summer in the northern hemisphere and this year will bring us a major highlight that won’t happen again until September 2035.
That highlight is a perihelic opposition of Mars on July 27. That’s when a planet is at its closest point to the sun at the same time it’s directly opposite the earth from the sun. The last time that happened for Mars was Aug. 27, 2003, when it was at its closest to earth in nearly 60,000 years, about the time modern humans first started migrating out of Africa, leaving genetic footprints that are still visible today. They were probably forced out during a sudden cooling period during the last ice age. That was a very dangerous time for humans because their numbers may have declined to as low as 10,000 worldwide.
We have sprung back to 7.5 billion, just over a billion people more than were around to see the last perihelic opposition just 15 years ago. Mars was only one million miles closer to Earth 15 years ago at 34.7 million miles than it will be this time. But this time it will be lower on the ecliptic in Capricorn than 15 years ago, so the views of many of its remarkable features through a telescope won’t be as easy to see.
Challenge yourself to see as much detail as you can this summer while Mars is still big and bright, because it will be 17 years until it gets this close again, allowing us to see it in some detail even without the use of a large professional telescope. Both of its icecaps, many of its dark markings and some of its thin atmosphere should be fairly easy to see in a good amateur telescope, but more exciting features like Olympus Mons – the biggest volcano in the solar system at three times the height of Mt. Everest with a base the size of Arizona and a 50-mile-wide caldera at the top will present more of a challenge. The volcanoes on Mars have been erupting for billions of years, and may have erupted as recently as 25 million years ago.
Another good challenge would be to find one or both of its moons, Phobos and Deimos, named for fear and terror. Phobos, at 14 miles in diameter, is the closer one to Mars. Deimos is the one farther out, making it slightly easier to find with a good telescope and a method to block out most of the light from Mars.
You would only weigh a few ounces on Deimos and its escape velocity is 12 mph, so you could run yourself into orbit around this tiny moon like Superman. On Phobos you could throw a baseball into orbit, since you only need to throw it at about 24 miles per hour.
Being the inner moon, Phobos is actually spiraling inward toward the Martian surface at the rate of about 6 feet per century, which is about half the distance that our own moon is drifting farther away from us every century. At that rate, Phobos will crash into Mars in about 40 million years and our own moon will be too far from Earth to create any more total solar eclipses in about 100 million years.
So enjoy this summer exploring the brilliant golden orange neighbor of ours, which still harbors many mysteries even as many more of its secrets are being revealed. We have already known that there was water on Mars at one time, but we have just recently discovered concrete evidence for large organic molecules just below its surface and possible microbial activity farther below its surface, causing seasonal variations in methane release. Mars could still be an active planet below its surface, hovering on the knife-edge of habitability.
The stories of the other planets will pale in comparison to the exciting possibilities of Mars this month, but they will provide a nice supporting cast and are always interesting to look at with or without a telescope, and to continue to learn more about them, since we really know very little about any of our seven other planets.
Venus is setting a little earlier again in our western evening sky, about an hour after sunset. But it’s still getting larger and brighter as it catches up with us in our orbits. It will be two magnitudes, or over six times brighter than Mars. Venus will appear like a waning gibbous moon through a telescope, shrinking from 70 percent to just 57 percent illuminated by the sun.
Venus will participate in two excellent close conjunctions this month, one with a planet and the other one with a star. Venus will be just one degree above and to the right of Regulus in Leo on the 9th, very close to where the sun was back on Aug. 21 of last summer when it was eclipsed by the moon. Then Venus will be less than one degree to the left of a thin waxing crescent moon on July 15 with Mercury below and to the right of the pair about 45 minutes after sunset.
Jupiter continues to fade a little this month as we leave it farther behind in space in our respective orbits. The king of the planets will end its retrograde or western motion in Libra on July 11. Jupiter won’t even get as bright as Mars this month, which is very unusual.
Saturn was at opposition last month, so it is now rising just before sunset and still visible all night long. Its golden glow is slowly fading, but it’s still brighter than usual and its rings are tilted open at 26 degrees, which is near its maximum. The ringed planet is still in retrograde in Sagittarius near the Lagoon and Trifid nebulae along an arm of our Milky Way galaxy.
Pluto is even at opposition this month, in the teaspoon in Sagittarius. Orbiting the sun once every 248 years, Pluto spends nearly 21 years in each zodiac constellation. It will only reach 14.8 magnitude this time, which is fully 2.5 million times fainter than Mars.
July 4: Henrietta Swan Leavitt was born on this day in 1868. She was an American astronomer at Harvard who discovered the period-luminosity relationship of Cepheid variable stars that allowed us to measure the universe and establish a distance scale.
July 5: On this day in 1687 Isaac Newton published his Principia Mathematica.
July 6: Earth is at aphelion, or farthest from the sun today at 94.2 million miles. Last-quarter moon is at 3:52 a.m.
July 9: Venus will be just one degree above Regulus, and 15 degrees above and to the left of Mercury tonight half an hour after sunset.
July 10: The slender waning crescent moon will be very close to Aldebaran in Taurus just before sunrise this morning. It will occult this red giant star in parts of Canada.
July 11: Mercury will be at its best low in the western evening sky after sunset tonight.
July 12: New moon is at 10:49 p.m.
July 14: The moon is just above Mercury this evening.
July 15: The moon will be very close to Venus this evening 45 minutes after sunset.
July 16: On this day in 1994, the first of 21 pieces of Comet Shoemaker-Levy 9 slammed into Jupiter, leaving a large black mark that lasted for a few weeks. I saw up to five of these marks at once over the course of six days as another piece descended into Jupiter’s atmosphere about every six hours.
July 19: First-quarter moon is at 3:53 p.m.
July 20: The moon is just above Jupiter tonight. On this day in 1969 the first humans, Neil Armstrong and Buzz Aldrin, set foot on the moon, just 66 years after the first primitive airplane was flown by the Wright Brothers.
July 24: The moon is just above Saturn this evening.
July 27: Full moon is at 4:22 p.m. This is also called the Hay or Thunder Moon.
July 30: Mars will be at its closest to Earth in 15 years tonight.
Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England.