Enjoying EarthSky? Subscribe.

102,316 subscribers and counting ...

Moon, planet Mars and star Regulus before dawn October 1


Tonight for September 30, 2013

Comet ISON to sweep closely past Mars on October 1

If you’re up before dawn on Tuesday, October 1, 2013 – when the sky is still good and dark, before dawn begins to wash the sky with its light – use the waning crescent moon to locate the red planet Mars and blue-white star Regulus, the brightest light in the constellation Leo the Lion. It’ll be fun to identify Mars on October 1, since this is the day the much-anticipated Comet ISON sweeps near Mars. Of course, you won’t see Comet ISON with your eye. It’s not nearly visible to the eye at this time. But it’ll be fun to imagine it up there!

What if you wait too late? If you wait until the time that morning dawn is washing the darkness from the sky, you probably won’t see Mars or Regulus either. In a dawn-washed sky, you’re only likely to see the three brightest celestial objects of the morning sky: the moon, the planet Jupiter and the star Sirius.

Keep your eye on Mars and the star Regulus for the next couple of weeks. Mars will meet up with Regulus for a close conjunction in mid-October 2013.

Bird's-eye view of inner solar system on October 1, 2013

Bird’s-eye view of inner solar system on October 1, 2013. The planets in their order going outward from the sun: Mercury, Venus, Earth and Mars

The inner solar system on April 14, 2014

Bird’s-eye view of the inner solar system on April 14, 2014, when Earth and Mars are at their closest until May 2016. Image credit: Solar System Live

Mars is relatively faint at present, though you should be able to see this world with the unaided eye in a clear predawn sky. Mars will remain a fixture of the morning sky for the rest of 2013. Moreover, it will climb higher into the predawn (and dawn) sky and will brighten all the while as the months go by. By the end of the year 2013, Mars will be twice as bright as it is now. It’ll be in the constellation Virgo, shining a touch more brilliantly than Virgo’s first-magnitude star, Spica.

But it’s really in the year 2014 that Mars will brighten by leaps and bounds. It’ll shine at its brightest best in April 2014, a whopping 16 times more brightly than it does right now. In April 2014, Mars will be about the same brightness as Sirius, the brightest star of the nighttime sky.

Astronomers of old were very confused by Mars’ dramatic change of brightness over the course of its approximate two-year sojourn in front of the constellations of the Zodiac. The astronomer Copernicus (1473-1543) solved the puzzle, explaining that the brightness variation was due to the fact that both the Earth and Mars orbit a central sun. The distance between Earth and Mars changes a lot over the course of two years. Therefore, the brightness of Mars varies with its distance from Earth.

For instance, tomorrow morning – on October 1, 2013 – Mars will be over 2 astronomical units away from Earth. In stark contrast, Mars will only be about 0.6 of one astronomical unit away in April 2014. So in April 2014, Mars will be less than one-third of its present distance from Earth.

By the way, one astronomical unit = sun/Earth distance = 93,000,000 miles or 150,000,000 kilometers.

Tomorrow, in the predawn darkness on October 1, use the moon to find the planet Mars and the star Regulus, the star depicting the heart of Leo the Lion.

Comet ISON to sweep closely past Mars on October 1

Everything you need to know: Comet ISON in 2013

Leo? Here’s your constellation