Twilight Scenes


Jupiter, Porrima, and the Moon at 10 p.m. EDT, June 30, 2017


Starting about 4:30 a.m. on Tuesday June 20 and Wednesday June 21, look for the beautiful crescent Moon in iconic pairings with brilliant Venus during the hour before dawn.

Summer begins in the northern hemisphere the morning of June 21 at 12:24 a.m. EDT. Then the Sun is directly over a point on the Tropic of Cancer at east longitude 114° 27', about 85 miles north-northeast of Hong Kong Island.

Star fields central in our early evening June sky provide views that are aside from bright constellations typical of later summer nights. Nevertheless there are many gems to behold. On these longest nights of the year the handle of the Big Dipper becomes visible high above the north-northwest horizon about an hour after sunset. This beautiful asterism’s curved handle guides our view southward toward zero magnitude Arcturus which, along with brilliant Vega rising in the east, are primary stellar beacons of June and early July evenings.

These evenings look toward the eastern sky for Vega, Altair, and Deneb forming the star pattern known as the summer triangle. Each is the brightest star of its constellation—Lyra, Aquila, and Cygnus, respectively. Vega is the brightest of this set with an apparent magnitude approximately the same as Arcturus. Both stars are among the most vivid in our entire sky.

In the constellation Lyra, near Vega, observers with binoculars or small telescopes may spot the so-called “double double”. It’s formally known as the star Epsilon Lyrae and, while small instruments show just two points of light, telescopes with magnifications over 100 may reveal a total of four component stars.

Also rising in the eastern sky, to the lower left of Vega, is blue-white supergiant star Deneb in the constellation Cygnus the Swan. Due to its great luminosity, Deneb is one of the most distant stars we can vividly see without optical assistance. It’s prominent in our sky even at the tremendous distance of about 1,600 light years. By comparison, Vega is just 25 light years from us.

The third corner of the summer triangle asterism is marked by first magnitude Altair in the constellation Aquila the eagle. It and its immediately adjacent stars Alshain and Tarazed form a small asterism called the “family of Aquila”. Altair’s distance from Earth is about 17 light years.

The waxing Moon returns to Jupiter’s vicinity in the southern sky on the evening of Friday June 30. The pair will appear during evening twilight with a five degree angular separation, and the span diminishes as hours pass. If your sky is clear it will be a good evening to telescopically explore a lovely scene—the nearly first quarter Moon, Jupiter, its four major satellites, and the star Porrima in the constellation Virgo.

Until the start of this century Porrima was a showpiece double star, separable into its components even in modest backyard telescopes. However, the pair was at closest orbital separation in 2007, and currently requires a substantial telescope to resolve. We’ll have to wait about half a decade before this binary again appears as two stars in most telescopes.

Third magnitude Porrima will be positioned just east of the dark lunar edge during early evening of June 30, later it will be occulted behind the eastward orbiting Moon at about 10:50 p.m. for observers in New York City. At about 11:55 p.m. EDT, the Moon moves past the star’s direction, but emerging at the bright lunar limb, seeing Porrima then will be a challenge. For times specific to your individual site, check specifics at Rob Robinson’s occultation website.

Looking forward, mark your calendar and get ready for the second Manhattanhenge of the year which takes place Thursday July 13 as the Sun sets at 8:21 p.m. EDT. But get to your viewing position about a half hour early to be ready. Check Director Neil deGrasse Tyson’s Manhattanhenge site for details.

Before we know it, the August 21 great American total solar eclipse will be upon us. If you haven’t made plans to select your observing site, book lodgings, visit relatives, or dust off tents and camping gear, check out NASA eclipse guru, and fellow Wagner College alumnus, Fred Espenak’s eclipse website, for his data packed interactive map.