Enjoying EarthSky? Subscribe.

173,505 subscribers and counting ...

Tonight

Cassiopeia the Queen after sunset

09sep13_430

The constellation Cassiopeia the Queen has the distinct shape of a W or M. Find her in the north-northeast sky on September and October evenings.

Mercury best for Southern Hemisphere now

2015-sept-4-mercury-spica-night-sky-chart

Mercury swings to its greatest evening elongation – 27 degrees east of setting sun – on September 4, 2015. But it’s low in the sky for observers on the northern part of Earth.

Capricornus? Here’s your constellation

Image credit of Capricornus the Sea-goat: Old Book Art Image Gallery

The constellation Capricornus the Sea-goat is best seen at early evening in September and October. If you live in the Northern Hemisphere, and are familiar with the Summer Triangle asterism, draw an imaginary line from the star Vega and through Altair to find this arrowhead-shaped constellation low in the southern sky. At mid-northern latitudes, the huge Summer Triangle asterism hangs high to the south to overhead on autumn evenings.

Moon sweeps through Taurus the Bull

2015-sept-3-aldebaran-pleiades-night-sky-chart

If you’re a night owl or early bird, watch the moon pass in front of the constellation Taurus the Bull over the next several days. Today’s sky chart shows the night of September 3-4. Information in this post about the occultation of Aldebaran by the moon on September 4-5.

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

09sep14_430

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

09sep26_430

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

2015-aug-31-mars-venus-night-sky-chart

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.