A Parade of Bright Planets and a Fantastic First Quarter Moon, the Equinox allows Aurorae, and Asteroid Vesta Veers into the Lagoon!

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(Above: Vincent Cheng of Hong Kong captured this long exposure image of Comet 21P/Giacobini-Zinner on September 15, 2018 using a DSLR on a tracking 70-mm telescope. It shows the comet hours before it passed through the bright star cluster Messier 25 (top centre). The bright golden star above the nebula at bottom right are the star Propus and the Jellyfish Nebula, which the comet will pass close to on Sept 16 and 17.)

Binocular Comet Update

I’ve been reporting on Comet 21P/Giacobini-Zinner, which returns to our vicinity every 6.5 years (hence the “P” for “periodic” in its designation). It reached its peak brightness last Monday, and it will now slowly to fade in brightness as it draws away from our vicinity. For the next few weeks, you should still be able to see it as a faint fuzzy greenish object in binoculars and small telescopes under a dark sky (away from city lights). You should be able to see its faint tail extending to the upper right of the comet (although your telescope will flip that direction around). Bright moonlight for the next two weeks will make viewing it a bit of a challenge, but give it a try.

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(Above: The path of Comet 21P/Giacobini-Zinner during this week, shown at 2 am local time. The red dots mark each midnight.)

The comet is located in the northeastern sky on a track that carries it lower every night — so we’ll have to stay up well after midnight, or get up before dawn, to see it. After it rises at 1:30 am tonight (i.e., early Monday morning), the comet will be positioned a finger’s width to the lower right of the bright star Propus, which marks the westernmost foot of Gemini’s (the Twins) legs. Another bright star named Tejat will sit a thumbs width to the left, so the comet will form a little triangle with those two stars. Every day the comet will drop a finger’s width lower.

Another comet, named 46P/Wirtanen, is predicted to become much bright enough to see without binoculars in late fall. Right now, it is tickling the belly of the whale (Cetus), but it’s far too dim to look for yet. Stay tuned!

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(Above: Northern autumn officially begins when the sun, travelling eastward along the ecliptic, crosses over the celestial equator on Saturday, September 22.)

Happy Autumnal Equinox!

At 9:54 pm Eastern Daylight Time on Saturday, September 22, the Equinox occurs, and autumn officially begins in the Northern Hemisphere. Here’s why…

At any given time, from our point of view here on Earth, the sun is situated in front of the distant background stars (although we can’t see them due to its bright glare). As Earth completes one orbit of the sun every year, the sun appears to shift eastward through those fixed stars, tracing a great circle around the sky. That circle is called the Ecliptic. Because Earth’s orbit is in more or less the same plane as all the other planets, the major Solar System bodies are always positioned near that imaginary track around the sky.

Now, imagine a second circle positioned directly over the Earth’s equator, and painted as a strip around the sky, and dividing the celestial sphere into two great bowls, the Northern Hemisphere stars and the Southern stars. Since the Earth’s equator is south of observers located in the Northern Hemisphere, that Celestial Equator circle around the sky is always in the southern part of the sky, and runs from the eastern horizon to the western horizon.

Because the Earth’s axis of rotation is tilted 23.5° away from the Ecliptic, the equator circle and the ecliptic circle behave like two hula hoops with the same centre, but with one tilted so that they intersect at only two spots. At the moment of the Autumnal Equinox, the sun is situated at one of the intersection points — and its apparent motion through the stars is carrying it into the southern bowl of the sky — as if it is “stepping over” the equator. Six months from now, on the Vernal Equinox, the sun will again cross the Celestial Equator at the other intersection point — this time heading into the northern bowl of the sky. At that moment, Spring will begin for the Northern Hemisphere.

The Equinox triggers a few interesting effects. First of all, for the next six months, the sun will spend all of each day in front of the southern hemisphere stars, and also high overhead of the lucky folks who live there! With the sun higher in their sky, they experience more daylight hours and receive more highly concentrated solar radiation, producing warmer weather. (Compare the intensity of a flashlight’s light when it’s beamed straight at a wall versus obliquely at the wall. The bright circle of the beam gets weaker when it spreads into an oval.) At the same time, North Americans, Europeans, and Asians have to accept shorter, colder days and longer nights (which are great for warmly dressed astronomers). On the day of the Equinox, everyone worldwide experiences 12 hours each of daytime and night-time. This is where the term, Latin for “equal night”, comes from.

The nights around the equinoxes offer better chances to see the aurorae at high northern and southern latitudes, too. Just as two bar magnets lined up with their poles in the same direction repel one another strongly, the Earth’s magnetic field repels the sun’s field. At the equinoxes, the Earth’s axis tilts neither towards nor away from the Sun, so the two “magnets” aren’t lined up as well, reducing Earth’s ability to deflect the Sun’s field and the charged particles that trigger aurorae in our upper atmosphere.

A final aside: The sun’s path along the Ecliptic is where the Zodiac originated. Early sky watchers noted that, in the course of one year, the sun travelled through twelve (well, 13 actually) constellations, passing through the same constellation every year on the same range of dates. Over the centuries, the dates have shifted somewhat. Nowadays, every year in the third week of September, the sun sits among the stars of the constellation Virgo (the Maiden).

The Moon and Planets

The moon’s First Quarter phase, when the half-illuminated moon will be sitting at a 90° angle from the sun, will occur worldwide tonight (Sunday) at 7:15 pm EDT. The name for this phase signifies that the moon has completed one quarter of its trip around Earth since the last New Moon. First quarter moons always set at midnight and rise at about noon, allowing the moon to be seen in the afternoon daylight, too.

(Above: The First Quarter moon, captured by Michael Watson of Toronto. His lunar gallery can be found here.)

During this week, pull out your binoculars or small telescope and aim at the moon. During this part of the moon’s orbit, sunlight is striking the moon at a very shallow angle, casting deep black shadows to the west (our left) of crater walls and mountain peaks. It also reveals subtle topographic features, especially on the floors of seemingly flat craters and dark maria (the Latin word for “seas”). The best sights are along the terminator — the dividing line between the lit and dark sides. This shadowed zone shifts west every night, highlighting new lunar geology.

After dusk on Monday evening, look for the first quarter moon sitting 4 finger widths to the upper left of medium-bright, yellowish Saturn. The pair of objects will fit into the same binocular field of view. They will cross the sky together until they set after midnight local time. On Wednesday night, look for the waxing gibbous moon sitting 4.5 finger widths to the upper right of bright reddish Mars. They will set after 2 am local time. For the rest of the week, the moon will pass through the water constellations of Capricornus (the Sea-Goat) and Aquarius (the Water-bearer), but its bright, nearly full disk will overwhelm those dim constellations’ stars.

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(Above: Venus sets in the west first, followed by Jupiter, as shown here for 8 pm local time on September 16)

Mercury is lost in the sun’s glare this week, and passes close to the sun on Thursday. Very bright Venus’ brilliance is allowing us to continue to see it while it sinks into the western evening twilight every night. Tonight, it will set at 8:30 pm, an hour after the sun. In a small telescope, Venus’ disk will resemble a waning crescent moon, lit only on the sunward side (although your telescope might flip the view). The planet is growing larger in apparent diameter because it is travelling towards the Earth right now. On Friday evening, Venus will reach visual magnitude of -4.78, its maximum brightness for this year.

Jupiter is being carried towards Venus by the westerly motion of the sky, but Venus is outpacing it as it swings towards the sun — so they won’t meet. This week, the two brightest planets are only about 1.5 fist diameters apart.

We are in the closing chapter for observing Jupiter this year. By the time the sky gets dark, the mighty planet will sit only a fist’s diameter above the southwestern horizon. It will set in the west-southwest at about 9:30 pm local time. Jupiter’s own orbital motion eastward will also continue to pull it away from the nearby bright star Zubenelgenubi, the brightest star in Libra (the Scales). In binoculars, you’ll plainly see that Zubenelgenubi is a pair of stars. While you have the binoculars handy, see if you can see Jupiter’s four Galilean moons (Io, Europa, Callisto, and Ganymede) flanking the planet.

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(Above: The moon will land near Saturn on September 17, as shown here at 10 pm local time. Mars sits off to Saturn’s left.)

Medium-bright and yellow-tinted Saturn will pop out of the darkening southern sky shortly after dusk this week, about 2 fist diameters above the horizon. The planet will set in the west by about midnight local time. This summer, the ringed planet has been 5 finger widths to the upper right of the “lid” star of the Teapot in Sagittarius (the Archer). As the sky darkens, even a small telescope should be able to show you some of Saturn’s larger moons, especially Titan. Using a clock’s dial analogy, Titan will move counter-clockwise this week from a position at 10 o’clock (above left of Saturn) to 4 o’clock (lower right of the planet). (Remember that your telescope might flip and/or invert the view. Use the moon to find out how your telescope changes things.)

For some time, Saturn has been sitting 2 finger widths to the upper left of two spectacular nebulas, the Trifid Nebula and the Lagoon Nebula. Late this week, the major asteroid Vesta will pass only one finger’s width below the Lagoon nebula.

Mars will still be a bright, orange-red colour this week. It will appear above the south-southeastern horizon after dusk and then climb higher until 10 pm local time, when it will reach an elevation of about 21° (or two outstretched fist diameters) above the southern horizon. (That will be the best hour to view the planet in a telescope because it will then be shining through the least amount of Earth’s distorting atmosphere.) Note that 20° is lower than many trees and buildings, so a clear southern vista is essential. It will set in the west a little after 2 am.

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(Above: The major asteroid Vesta at bottom centre, will pass near the beautiful Lagoon Nebula on the nights surrounding September 21. Saturn will remain to the upper left of the Trifid Nebula once Vesta moves on.)

At visual magnitude 5.7, blue-green coloured Uranus is visible from mid- evening until dawn. You can see it without optical aid under very dark skies, or in binoculars and telescopes under moderately light-polluted skies. During late evenings, the ice giant planet is located in the eastern sky, about 4 finger widths to the left of the modestly bright star Torcular (Omega Piscium). That star sits a generous palm’s width above the “V” where the two starry cords of Pisces (the Fishes) meet.

Distant Neptune recently reached opposition, when it is visible all night and closest and brightest for this year. Using a decent quality telescope you can see the very blue planet among the dim stars of Aquarius (the Water-bearer). Look for the magnitude 7.8 planet sitting midway between the modestly bright star Phi (φ) Aquarii and the brighter star Hydor (Lambda Aquarii). It will highest in the sky (best viewing conditions) at about 12:30 am local time.

Astronomy Skylights for this week (from September 16th, 2018) by Chris Vaughan. Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.

Point your device at the sky and see what stars, constellations, and satellites you are looking at 🌌✨ https://starwalk.space

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