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Tonight

How Earth looks from outer space

Here is Earth from 900 million miles away, from the vantage point of the rings of Saturn.  Image via the Cassini spacecraft, which has been orbiting Saturn since 2004.

Here is Earth from 900 million miles away, from the vantage point of the rings of Saturn. Image via the Cassini spacecraft, which has been orbiting Saturn since 2004. This image was acquired by Cassini on July 19, 2013.

How far away from Earth do we have to go to not see it with our own eyes? To answer this question, you have to take into account how brightly Earth reflects sunlight. And the sun itself is another important factor. As seen from any great distance, Earth appears right next to the sun – and, from a great distance, the glare of our local star would make Earth difficult or impossible to see. So imagine blasting off in space and looking back toward Earth. How far away could you be, and still see it?

Use Big Dipper to find North Star

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Tonight’s chart shows Polaris and the Big and Little Dippers for a September evening. You can use the Big Dipper to find Polaris, which is also known as the North Star. Notice that a line from the two outermost stars in the bowl of the Big Dipper points to Polaris. And notice that Polaris marks the tip of the handle of the Little Dipper.

Star of the week: Alpha Cephei, a rapidly rotating star

Astornomers used the CHARA array to learn the inclination, polar and equatorial radius and temperature, as well as the fractional rotation speed of Alpha Cephei.  Read about this work here.

Image of Alpha Cephei from the CHARA array at Georgia State University.

The constellation Cepheus the King is not terribly conspicuous and can boast of only one relatively star. That star is Alderamin – aka Alpha Cephei – which is by far the brightest star in Cepheus, lighting up one corner of an otherwise faint house-shaped pattern of stars. While not one of the most conspicuous stars in the night sky, this star is easy to spot, and it is interesting for its rapid rotation on its axis. Follow the links inside to learn more about Alderamin, aka Alpha Cephei.

How to see the Summer Triangle in September

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The Summer Triangle consists of three bright stars in three separate constellations. The stars are Vega in the constellation Lyra, Deneb in the constellation Cygnus, and Altair in the constellation Aquila. The Summer Triangle is prominent on summer evenings, but now, as autumn begins, we still have several months to watch this huge star pattern that looms from south to overhead in the autumn evening sky.

September 2015 guide to the five visible planets

Skywatcher, by Predrag Agatonovic.

Skywatcher, by Predrag Agatonovic.

At mid-northern latitudes, Saturn lords over the evening sky all by himself all month long! Mercury is hiding in the evening twilight for the Northern Hemisphere. Meanwhile, at southerly latitudes, two planets are visible in the evening, as Mercury presents its finest evening apparition of the year. The other three visible planets – Venus, Mars and Jupiter – are in the east before sunrise, with Venus pointing the way to Mars and Jupiter appearing in mid-month in predawn twilight. Follow the links inside to learn more about September planets.

Use Venus to find Mars in September

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They’re both up before the sun. Mars is faint and not very noticeable, but Venus is bright! It outshines Mars by a few hundred times. They’ll be easier to catch from the Northern Hemisphere than the Southern Hemisphere. From everywhere worldwide, the view of these two morning worlds will steadily improve throughout the month.

Neptune closest to Earth for year on August 31

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Neptune comes closest to Earth today – on August 31, 2015 – and reaches opposition less than a day later, on September 1. By closest, we don’t mean close. Neptune lodges in the outskirts of our solar system. At opposition, this giant world lies 29 times farther away from Earth than Earth lies from the sun.

Astronomy events, star parties, festivals, workshops

Here's the Texas Star Party in 2009, one of the biggest such events of the year, drawing about 500 deep-sky enthusiasts and their telescopes to the Davis Mountains of West Texas.  Image via Todd Hargis / Ron Ronhaar.  Used with permission.

Here’s the Texas Star Party in 2009, one of the biggest public astronomy events of each year, drawing about 500 deep-sky enthusiasts and their telescopes to the Davis Mountains of West Texas. Image used with permission, via Todd Hargis and Ron Ronhaar.

Interested in astronomy, but not sure where to begin? Seek out your local astronomy club, a roomful of willing and able amateur astronomers, whose telescopes may offer your first glimpse of the cosmos. The Astronomical League, an umbrella organization of 240 amateur astronomy clubs and societies in the U.S., helps us create and maintain the list of events on this page.

Moon swings to perigee – closest point – on August 30

The moon's apparent size in our sky depends on its distance from Earth.  The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left).  Image by Marco Langbroek of the Netherlands via Wikimedia Commons.

The moon’s apparent size in our sky depends on its distance from Earth. The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Image by Marco Langbroek of the Netherlands via Wikimedia Commons.

The moon reaches perigee – nearest point to Earth in its orbit – on August 30, 2015 at 15:24 UTC. This perigee comes less than one day after the moon reached the crest of its full phase on August 29 at 18:35 UTC.

First of 3 full supermoons on August 29

Image credit: Stefano Sciarpetti

Image credit: Stefano Sciarpetti

Supermoon weekend! The full moon of August 29, 2015 will be the first of this year’s three full supermoons. It’s a full moon near perigee, or near its closest point to Earth for the month. Like it or not, modern skylore dictates that these sorts of moons are called supermoons.