Summer is in full swing now and there will be several interesting highlights to observe. The nights are at their shortest for the year now, and the twilights at their longest.

It doesn’t get completely dark until 10:48 p.m. at the beginning of the month, and even by the end of the month it won’t be completely dark until 10:08 p.m.

Additional Photos

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 Mercury 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

Full night doesn’t start until the end of astronomical twilight, when the sun gets to 18 degrees below the horizon. There are two other levels of twilight, called civil and nautical twilight. Each is defined by the sun being an additional 6 degrees below the horizon.

Civil twilight begins as soon as the sun sets below the horizon and lasts until it reaches 6 degrees below the horizon. Some of the brighter stars and planets become visible during nautical twilight, which lasts from the time the sun sinks from 6 to 12 degrees below the horizon. By the end of nautical twilight the horizon is no longer visible and details on objects are no longer discernible.

Astronomical twilight lasts from the time the sun reaches 12 degrees below the horizon until it reaches 18 degrees. You may think it is completely dark by the beginning of astronomical twilight, but most of the fainter and more subtle aspects of the beauty of the night sky, like many nebulae and galaxies, don’t show themselves until the end of astronomical twilight, when the night really begins.

Surprisingly, there are many large cities not that far north of us that never get out of astronomical twilight at all this time of year. They include Kiev, Warsaw, Prague, Berlin, Amsterdam, London and Vancouver. They are all just over 50 degrees north latitude, about 7 degrees farther north than southern Maine.

There are many interesting phenomena to watch for during the long twilights we are experiencing, or the “blue hour,” as it has been called by artists.

You can actually see the earth’s shadow projected onto our own atmosphere in fairly good detail for about 15 minutes around the beginning of civil twilight. The striking, broad, low arch of pinkish light that will grow just above the shadow of the earth is called the belt of Venus. It will dissipate quickly, as the shadow keeps darkening dramatically, until it slowly covers the whole sky and finally begins to reveal the greater universe that is well hidden by the sun during the day.

Most of you have probably seen this many times, but have not really been aware of it or known exactly what causes it. This brings to mind a quote from Henry Beston’s “The Outermost House”: “As the great Earth, abandoning day, rolls up the deeps of the heavens and the universe, there opens a new door to the human spirit, and few there be so clownish that some awareness of the mystery of being does not touch them as they gaze.”

Other interesting events you may witness during this magical hour include sun pillars, crepuscular rays and even Noctilucent clouds, which look a little like northern lights and are caused by sunlight reflected off the highest clouds above the Earth, about 50 miles up in the mesosphere. They are usually visible only between 50 to 70 degrees north latitude, because these are polar clouds.

There are two triangles now in the sky after it gets dark. The first is the famous summer triangle, which consists of Vega in Lyra, Deneb in Cygnus, and Altair in Aquila.

These stars are easy to find because they are among the brightest stars in the summer sky. One arm of our Milky Way galaxy runs right through the middle of this triangle and splits apart in the Cygnus star fields.

The next triangle is only temporary, since it includes the planet Saturn, which is always moving relative to the fixed background of stars. Look low in the western sky around 10 p.m., and you will see that golden Saturn is still very close to Porrima, a double star in Virgo. Then look about 15 degrees to the left of Saturn to see Spica, the brightest star in Virgo.

Then complete the triangle by going up about 30 degrees to Arcturus, a bright orange star that is easy to find. Just follow the arc of the Big Dipper down towards the southwest. The saying goes, “arc to Arcturus and speed on to Spica.”

Mercury will join Saturn in the evening sky for the first half of July. Look for a slender waxing crescent moon passing below and to the left of Mercury during the evenings of July 2, 3 and 4. As the moon gets fuller and towards first quarter, watch it pass below Saturn on July 6, 7 and 8.

Jupiter rises around 2 a.m. at the beginning of July and will rise at midnight by the end of the month. Watch a last quarter moon pass just above the king of the planets in the eastern sky around 2 a.m. July 22 and 23.

Mars is in Taurus now and rises a couple of hours before the sun. Watch the waning crescent moon pass below the Pleiades and then Mars on the mornings of July 25, 26 and 27.

Venus rises only about one hour before the sun. Venus looks full through a telescope now since it is near superior conjunction.

JULY HIGHLIGHTS

July 1: New moon is at 4:54 a.m. EDT.

July 4: On this day in 1054, the supernova that is now the Crab nebula in Taurus became visible to us on Earth. The Earth is at aphelion or furthest from the sun today at 94,512,000 miles, or just 3.3 percent greater than its minimum distance on Jan. 4.

July 8: First quarter moon is at 2:29 a.m.

July 11: Antares in Scorpius, one of the biggest stars in our Milky Way, at 700 times the sun’s diameter, will be just 2 degrees below the moon.

July 12: Neptune will complete one full orbit around the sun since its discovery in 1846. The first planet to be predicted mathematically before it was actually seen, this is a fascinating story involving nine astronomers from three countries. It takes 165 years to orbit the sun.

July 15: Full moon is at 2:40 a.m. This is also called the Thunder or Hay Moon.

July 20: Apollo 11 landed on the moon on this day 1969, with Neil Armstrong and Buzz Aldrin becoming the first two humans to ever set foot on any place other than Earth.

July 23: Last quarter moon is at 1:02 a.m.

July 28: The first photo of a solar eclipse was taken on this day in 1851. The Delta Aquarid meteor shower peaks tonight and the next night. You can only expect around 20 meteors per hour, but some early Perseids will already be visible.

July 29: On this day in 2005, the possible 10th planet, Xena, was discovered. It has been renamed Eris and is about the same size and mass as Pluto, which now is also just a dwarf planet, or Kuiper belt object or trans-Neptunian object.

July 30: On this day in 1971, Apollo 15 landed on the moon. New moon is at 2:40 p.m.

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

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