Now that summer has settled across our hemisphere and the nights are slowly getting longer again, there are several interesting celestial highlights this month.

A good way for us to become more aware of our place in the solar system — and the larger-scale motions our spaceship Earth goes through as it ceaselessly orbits the sun while spinning on its axis — is by carefully watching our neighboring planets.

You will get an excellent chance to do that as four of the five brightest planets will all be getting closer together in our evening sky during the last couple of weeks of the month.

Our two next-door neighbors, Venus and Mars, along with Saturn, form a nice, downward-slanting line in the western evening sky one hour after sunset that covers 38 degrees, or nearly four fists at arm’s length. the middle of July, that line will shrink to 24 degrees, and by the end of the month, it will turn into a triangle, with Mars and Saturn less than 2 degrees apart and Venus less than 8 degrees away.

Mercury will join this trio starting in mid-July. So this line of planets will shrink by a factor of five over the course of these 31 days.

This is also a great time to become more aware of the ecliptic and how its angle to our horizon slowly changes. Just sweep your arm along this path and include the moon if it is up, which it will be during the second half of this month.

If you have a good horizon and a clear sky, you will be able to see six of the 12 zodiac constellations at any one time. You should now be able to see Venus and Mars in Leo, Saturn in Virgo, and then continue following this path through Libra, Scorpius, Sagittarius and Capricornus. As Leo sets, notice that Aquarius will rise, and then Pisces as Virgo sets.

This ecliptic line defines the plane of our solar system and all the planets, the sun and the moon always have to travel on this path because our sun has 99.99 percent of the mass of our solar system, which forces all the planets to orbit it in nearly the same plane.

As you watch these four planets gather for a nice conjunction, this is a good time to remember how fast each planet is actually orbiting the sun. All the planets and moons have to follow Kepler’s laws, so you would expect Mercury to orbit the fastest and Neptune the slowest.

Mercury orbits at 30 miles per second, Venus at 22 m/sec, Earth at 18.6 m/sec, Mars at 15 m/sec, Jupiter at 8 m/sec, Saturn at 6 m/sec, Uranus at 4.2 m/sec and Neptune at only 3.4 m/sec.

As you recognize that all planets in all solar systems have to follow these laws, also understand that these laws do not apply on the galactic scale. All the stars have to orbit around the center of every galaxy at nearly the same speed. First discovered about 40 years ago, a mysterious substance called dark matter allows that to happen. Otherwise galaxies would have wound themselves up and collapsed a long time ago.

It turns out that about 90 percent of every galaxy is dark matter and only 10 percent of it is made up of the visible stars. You could compare that to an iceberg, with only about one tenth of it being visible above the surface of the water. There are several good theories as to what dark matter could be and many good experiments hoping to find the first dark matter particle, but nothing has been found yet.

On the scale of the whole universe, about one third is dark matter and about two thirds of it is dark energy, which is an even more mysterious force, just discovered in 1998, which is forcing the ever-increasing rate of expansion of our whole universe. That leaves only 4 to 5 percent of the entire universe being composed of stars and luminous matter.

So now you can compare everything you can see in the universe as being just the tip of the iceberg, or only a small percentage of what is really there all the time.

Other interesting things to be aware of as you watch this dramatic conjunction slowly unfolding all month long is the newly discovered phenomenon happening on some of these planets and their moons. We just found compelling evidence for active volcanoes on Venus, our sister planet, which is the same size as Earth but has very hostile conditions, with a constant surface temperature of 850 degrees F. Venus is even hotter than Mercury, which is closer to the sun, and has a pressure 100 times that of the surface air pressure on Earth, which would feel like being 3,000 feet under our oceans.

We just discovered that Saturn has a new giant storm going on that is even visible in amateur telescopes from Earth. Then we also discovered recently that Jupiter lost one of its two brightest cloud bands, called the South Equatorial Belt, where the great red spot resides. That belt does tend to fade out in cycles from three to 15 years and it is probably obscured by clouds rather than actually having disappeared, but it is still dramatic to see Jupiter with only one bright cloud band.

You can check that out for yourself, as the king of the planets now rises around midnight in Pisces, which is about the time that Saturn will set.

As you watch Venus and Mars catching up with Saturn this month, notice that during the evening of Friday, July 9, Venus will pass just 1.1 degrees above Regulus, the brightest star in Leo and the 21st brightest star in the whole sky. Last month, Mars also passed very close to Regulus on the ecliptic. Mercury joins the trio on July 13.

Watch the slender waxing crescent moon point out each of these four planets one by one over the course of these four days.

On Wednesday the 14th, the moon will be near Venus, then it will be near Mars on the 15th, and finally the moon will pass under Saturn on the evening of the 16th.


July 4. Last quarter moon is at 10:35 a.m. EDT. Jupiter is about 10 degrees to the lower right of the moon in the morning sky.

July 6. The Earth is at aphelion, or farthest from the sun for the year, at 94,508,000 miles, or just 1.7 percent farther than the average distance of 93 million miles.

July 8. The Pleiades star cluster is just above the thin waning crescent moon in the east one hour before sunrise.

July 9. Regulus is one degree below Venus this evening and the next.

July 11. New moon is at 3:40 p.m. There will be a total eclipse of the sun visible over Easter Island and parts of the South Pacific today.

July 13-16. The waxing crescent moon passes below Mercury, Venus, Mars and Saturn over these four evenings.

July 18. First quarter moon is at 6:11 a.m.

July 20. On this day in 1969, the first humans ever to walk on the moon were Neil Armstrong and Buzz Aldrin. We only landed there five more times until December 1972, and have not gone back since then.

July 25. Full moon is at 9:37 p.m. This is also called the Hay or Thunder Moon.

July 28. The first photo of a total solar eclipse was taken on this day in 1851. Mars is less than 2 degrees below Saturn on this evening and the next. 

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