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Why sun rises due east and sets due west at the equinox

Where does the celestial equator intersect your horizon?  No matter what you latitude is, it intersects your horizon at points due east and due west.  Read more about how an observer's latitude affects your visible sky.

Where does the celestial equator intersect your horizon? No matter what your latitude is, it intersects your horizon at points due east and due west.

The September 2014 equinox happens on September 23 at 2:29 Universal Time, which translates to September 22 at 9:29 p.m. Central Daylight Time for us in the central U.S. So as you read this, the exact moment of the equinox may have already happened for you. It’s often said that – at each equinox – the sun rises due east and sets due west. And that’s true. But why? How can you conceptualize this?

Sun at zenith over Earth’s equator on the September equinox

Simulated image above: Planet Earth, from the vantage point of the sun, at the instant of the September 2014 equinox.

Simulated image above: Planet Earth, from the vantage point of the sun, at the instant of the September 2014 equinox.

At the instant of the September equinox, the midday sun will be shining straight overhead at the equator. As the September equinox sun crosses the equator, going from north to south, it’ll signal the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.

Everything you need to know: September equinox

Equinox sunset 2012 from EarthSky Facebook friend Jenn Moon in Lakewood, Colorado.

The 2014 September equinox comes on September 23, at 2:29 Universal Time. Although the equinox happens at the same moment worldwide, the clock times will vary by time zone. So in the U.S. this equinox comes on September 22 at 10:29 p.m. EDT, 9:29 p.m. CDT, 8:29 p.m. MDT or 7:29 p.m. PDT.

Hazy pyramid of light in east? False dawn.

Here's the zodiacal light as captured on film in Canada.  This wonderful capture is from Robert Ede in Invermere, British Columbia.

Here’s the zodiacal light as captured on film in Canada. This wonderful capture is from Robert Ede in Invermere, British Columbia.

Late summer and early autumn present the best time of year to see the false dawn, also known as the zodiacal light. With the moon out of the morning sky for the next two weeks, this is your chance to see the pyramid-shaped glow in the east before dawn. It’s even “milkier” in appearance than the starlit trail of the summer Milky Way.

Mercury’s September 2014 apparition favors Southern Hemisphere

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Mercury, the innermost planet of the solar system, swings to its greatest evening elongation (26o east of the setting sun) on September 21, 2014. Then, in a little more than one day thereafter, it’ll be the September equinox. So in September 2014, Mercury’s greatest evening elongation pretty much comes concurrently with the September equinox.

The most favorable time to see Mercury in the evening sky is when this planet’s greatest eastern (evening) elongation closely coincides with the spring equinox. Because the September equinox is the Southern Hemisphere’s spring equinox, southerly latitudes get to see a wonderful evening apparition of Mercury throughout the month of September. However, in the Northern Hemisphere, the September equinox is the autumn equinox. Unfortunately, Mercury’s presence in the evening sky is most subdued when its greatest evening elongation happens in close conjunction with the autumn equinox. That’s why the Southern Hemisphere has it over the Northern Hemisphere for this particular evening apparition of Mercury.

Moon and Jupiter spectacular before dawn September 20

2014-sept-19-moon-jupiter-regulus-night-sky-chart

The waning crescent moon and the brilliant planet Jupiter still adorn the eastern predawn and dawn sky on September 20. If you’re up before dawn, you should be able to spot the star Regulus below the moon and Jupiter. Will you also see Venus? Maybe.

Moon and Jupiter before dawn September 19

2014-sept-18-moon-jupiter-night-sky-chart

Watch for the waning crescent moon near the dazzling planet Jupiter during the predawn and dawn hours on Friday morning – September 19, 2014 – and for a morning or two after that. Simply look eastward to behold these two glorious worlds reflecting the light of the sun.

Sun passes out of Leo and into Virgo on September 17

2013-sept-16-virgo-leo-sun-night-sky-chart

Today – September 17, 2014 – the sun exits the constellation Leo and enters the constellation Virgo. The sun will remain in front of the constellation Virgo the Maiden until it enters the constellation Libra the Scales on October 31. So, when the equinox arrives on September 23, the sun will be in front of the constellation Virgo. And so it has been for thousands of years. But it won’t always be that way.

Year’s fastest sunsets happen around the equinoxes

Sunrise over the Red Sea by EarthSky Facebook friend Graham Telford

Photo by EarthSky Facebook friend Graham Telford

Does the sun set faster around the time of an equinox? Yes. The fastest sunsets (and sunrises) occur at or near the equinoxes. What’s more, the slowest sunsets (and sunrises) occur at or near the solstices. This is true whether you live in the Northern or Southern Hemispheres. And, by the way, when we say sunset here, we’re talking about the actual number of minutes it takes for the body of the sun to sink below the western horizon.

Andromeda galaxy is Milky Way’s next-door neighbor

The Andromeda Galaxy with two of its satellite galaxies, via Wikimedia Commons. Click here to expand.

Excluding the Large and Small Magellanic Clouds (which can’t be seen from northerly latitudes), the Andromeda galaxy – also known as M31 – is the brightest galaxy in all the heavens. It’s the most distant thing you can see with your unaided eye, at 2.3 million light-years. To the eye, it appears as a smudge of light larger than a full moon. Follow the links inside to learn more about the Andromeda galaxy.