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The Sky This Week, 2018 July 24 - 31

Red Moon, Red Planet.
Jupiter with the four Galilean moons, imaged 2018 July 20

The Moon brightens the evening sky this week as she waxes to the Full phase, which occurs on the 27th at 4:20 pm Eastern Daylight Time.  July’s Full Moon is popularly known as the Hay Moon, Elk Moon, Buck Moon, or Thunder Moon.  The latter seems particularly appropriate for the weather we’ve had in Washington lately!  This Full Moon will be a special one for central Asia, Europe, and Africa, where residents will see the longest total lunar eclipse of the Century.  The reason for the unusual length, 1 hour 43 minutes of totality, is that the Moon is at its second most-distant apogee of the year.  This means the Moon is moving at its slowest speed in its orbit.  It is also passing very close to the geometric center of the Earth’s umbral shadow.  Fortunately, you don’t have to make last-minute plans to travel to Madagascar to see it.  If you can hang on until January 20, 2019 you’ll get a ringside seat to the next total lunar eclipse, which will be visible from the entire United States.

The bright Moon washes out the summer Milky Way and the fainter stars of the summer constellations for most of the week.  You can still enjoy an active sky, though, with four planets and a bevy of bright stars available for your enjoyment.  High in the west as darkness falls is an old friend from the springtime, the bright star Arcturus.  The brightest star in the northern hemisphere of the sky, Arcturus is the fourth-brightest star in the heavens.  It is bright because it is relatively close to us at a distance of just under 37 light-years.  It is also a “red giant” star, one that has evolved away from fusing hydrogen into helium in its core.  Instead, the fusion reactions take place in a shell surrounding a helium core.  This is probably a very good example of what the Sun will look like in another few billion years.  Arcturus has swollen to a diameter about 25 times larger than the Sun, and it shines with a luminosity of about 170 Sols.  It’s pale red tint contrasts nicely with the white and blue-white stars of the Summer Triangle, now high in the east during the late evening.  Two of the three stars of the Triangle are actually closer to us than Arcturus.  The brightest, Vega, is about 25 light-years away, while Altair, the southernmost, is a mere 16.7 light-years distant.  Deneb, the easternmost and faintest of the three, is a star of completely different nature.  It is a very hot “blue giant” star that is over 100 times the distance to Altair.  Its luminosity is hundreds of thousands of times greater than that of the Sun.  If we could somehow drag it to the distance of Altair it would shine with the apparent brightness of the first-quarter Moon, easily casting shadows through the night.  It would also be a significant source of ultraviolet radiation, so we’d need to slather SPF-50 sunscreen on ourselves whenever it is above the horizon!

Venus continues to hover over the western horizon during the fading evening twilight.  She is gradually starting to settle toward the horizon from night to night as she starts to swing between the Earth and Sun on her faster, inner orbit.  She is also moving southward along the ecliptic, and next week she will slide into the southern hemisphere of the sky as she drifts toward the stars of the springtime constellation of Virgo.

Jupiter has resumed direct west-to-east motion along the ecliptic and is gradually creeping up on the second-magnitude star Zubenelgenubi, one of the two bright stars of the constellation Libra.  He’s now well west of the meridian when the sky darkens, and you’ll only have a limited time to catch a glimpse of him through the telescope before he settles into the denser air layers above the horizon.  If you have a four-inch telescope, look for the famous Great Red Spot rotating across his cloud-striped disc on the evenings of the 26th and 28th.  You can enjoy the antics of his four bright Galilean moons on any night.

Saturn crosses the meridian at around 11:00 pm local time.  This is the best time to view the ringed planet, when Saturn is highest above the southern horizon.  Unfortunately, Saturn is about as far south in his 29-year journey around the Sun as he can get, so you’re viewing him through lots of our atmosphere.  Wait for brief moments of steady air to look for details in the planet’s rings.

Mars reaches opposition on the 27th.  This is when he is exactly opposite the Sun in the sky, rising at sunset and setting at sunrise.  At the same time as opposition he will be accompanied by the eclipsed Full Moon, so there will be two red objects for viewers to enjoy from the other side of the world.  For amateur astronomers this is the best time to observe the red planet; his apparent diameter is over 24 arcseconds which will allow modest telescopes to resolve some of his surface features.  Unfortunately, there is still a planet-girdling dust storm going on, so most of us are seeing a pale pink featureless dot.  Hopefully both the local terrestrial and global martian weather will improve soon!

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