Here comes summer! The summer solstice comes on June 20 this year, when the Sun is as far north as it gets. Nights will be warmer, but you need to stay up later to see the stars! Sunset does not come until around 9 p.m. in June, and skies are not completely dark until much later, about 11:45 p.m. on June 20. Don’t let that stop you though, as stars and constellations are visible much earlier.
That time after sunset, when the sky is still partly illuminated, is called twilight. The corresponding term for the morning is dawn. Astronomers divide twilight into three periods. The first is called “civil twilight,” when the Sun has set but is less than 6 degrees below the horizon. Civil twilight ends at about 9:40 p.m. in June. During civil twilight, there is enough natural sunlight that artificial light is not needed for outdoor activities, and you can only view the brightest objects in the sky, like the bright planets and bright stars (and the Moon, of course).
Nautical twilight comes next, when the Sun is between 6 and 12 degrees below the horizon. In June, nautical twilight will end at about 10:30 p.m. Artificial light is usually needed for outdoor activities, and most stars can be seen with the naked eye. The term comes from times when sailors used the stars to navigate. The final stage of twilight is called astronomical twilight, when the Sun is between 12 and 18 degrees below the horizon. Most objects can be viewed with a telescope during astronomical twilight, although a small amount of sunlight still scattered in the sky may make faint objects difficult to see. Astronomical twilight ends at around 11:45 p.m. in June. In the morning, the three twilight zones are reversed. Astronomical dawn comes at about 2:30 a.m., nautical dawn at about 3:45 a.m., and civil dawn starts at about 4:30 a.m.
Venus and Mars continue to be the visible planets in the evening sky. Last October, when Mars was at opposition, the red planet was about 39 million miles from Earth. At the start of June, Mars will be about 209 million miles away, and will be some 225 million miles from us at the end of June. It is easy to see why the Mars Rover Perseverance was launched in the fall of 2020! Venus is much closer at about 150 million miles in June. Venus is easy to spot after sunset, as the bright “evening star,” low in the west.
If you are up before sunrise, you can see Saturn and Jupiter as bright “stars” low in the southern sky. On June 1, the waning gibbous Moon was right below Jupiter. On June 27, the Moon will be just below Saturn, and will be below Jupiter again on the 28th. June’s new Moon will come on June 10, with full Moon following on the 24th.
On June 10, there will be an annular eclipse of the Sun. Alas, it will not be visible in our area, occurring from about 2:30 a.m. to 4 a.m., when the Sun is below the horizon. “Annular” eclipses occur when the Moon is slightly farther away from Earth, and the apparent size of the Moon is a bit smaller than the Sun. The Sun appears as a bright ring around the dark disk of the Moon. Another term for a ring is an annulus, hence the name. It does not mean it occurs annually!
Last month I mentioned the bright star Vega, the fifth brightest star in our sky. I also briefly mentioned nearby Arcturus, which comes in at No. 4, slightly brighter than Vega. You can locate Arcturus by following the “arc” of the Big Dipper’s handle to a bright star, Arcturus. The star was an interesting focus for the 1933 World’s Fair in Chicago. A previous World’s Fair in Chicago had occurred in 1893, 40 years before. Arcturus was about 40 light-years from Earth, so light arriving in 1933 would have left the star in about 1893. Telescopes were used to focus the star’s light on photovoltaic cells, and the resulting electric current was used to flip a switch, turning on the lights for the Fair. We’ve since refined the distance to Arcturus to be about 37 light-years, but it was a unique use of technology for the time.
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