Thursday, July 3, 2008

I Love A Parade

One of the best parades you can see this Independence Day Weekend won’t involve marching bands and won’t require street closures. This parade will take place in the night sky.

Moving in tight formation throughout our observing week will be the waxing (growing) crescent Moon, planet Mars, planet Saturn, and the Lion’s Heart, the brightest star in the constellation Leo the Lion.

Celestial alignments like this one give us a glimpse into the clockwork of the universe. The changing phases of the Moon remind us we’re part of a three-body orbital system: Earth, Moon, and Sun. If we don’t remember to look up occasionally, it’s easy to forget that everything in the universe is in motion, all the time.

To see the alignment, you’ll need to face west, the direction where the Sun sets. Start observing right after sunset. The first object you’ll spot is the crescent Moon. Your ability to spot stars in evening twilight will determine how long it takes you to spot the other three objects. The planets will look like bright stars. Because of their order of brightness, you’re likely to spot Saturn next, then the Lion’s Heart, and finally Mars when the sky darkens enough.

1) On Friday the 4th, the Moon will set about an hour and a half after sunset, so you’ll need to snag it quickly. Its distance above the horizon at sunset will be about the height of your fist. Make a fist and hold it at arm’s length in front of you, with your thumb at the top. Place the bottom of your fist at the horizon or about where you think the horizon is, if it’s obscured by land forms, vegetation, or buildings. Look for the thin crescent Moon at or slightly above the top of your fist.

When you spot it, be sure to look for earthshine. Earthshine is sunlight reflecting off the surface of the Earth and illuminating the dark part of the Moon. Even though the bright crescent is the only part of the Moon’s face being directly illuminated by the Sun, if you look carefully, you’ll see that the rest of the Moon’s face is glowing faintly with reflected glory: earthshine.


Western sky at sunset on July 4
All sky charts created with
Your Sky


About a fist and a half above and to the left of the Moon will be the star Regulus, the Lion’s Heart. Regulus (pronounced REGG-yoo-luss) is Latin for little king. This regal star marks the position of the celestial king-of-the-jungle’s heart. Mighty Regulus is 140 times brighter than our Sun.

Continuing on that same diagonal line, just beyond Regulus will be reddish Mars, and just beyond Mars will be golden-colored Saturn.

2) Saturday the 5th will boast the best alignment of the week. This is the one to observe, if your weather permits. The Moon will scoot up to close the gap with Regulus, and you’ll now find the crescent about two fists above the horizon. It’ll be quite a spectacle to see the Moon, Regulus, Mars, and Saturn in a neat diagonal line, close to one another. This isn’t an everyday occurrence!


Western sky at sunset on July 5

Keep in mind that this apparent closeness is an optical illusion. Although we can’t see it, the night sky is three-dimensional. The Moon is only a quarter of a million miles from Earth, Mars is now 200 million miles away, Saturn is now 750 million miles away, and Regulus--the only alignment object not in our solar system--is 77 light years away. One light year is the distance light travels in one Earth year, nearly six trillion miles. Multiply that times 77, and you get a number that makes your head hurt.



Western sky at sunset on July 6

3) On Sunday the 6th, the Moon will break formation and will pull up next to Saturn and Mars. Remember, the Moon is always revolving around Earth from west to east, completing one orbit in about a month (29.5 days). So each night at the same time, the Moon will have moved a bit to the east.




Western sky at sunset on July 7

4) On Monday the 7th, the Moon--now 26% illuminated--will continue on its journey east, pulling past the huddle of Regulus, Mars, and Saturn. You may also notice the gap between Mars and Saturn beginning to close.




Western sky at sunset on July 8

5) On Tuesday the 8th, the Moon will be even higher in the sky at sunset. Mars continues to close in on Saturn.



Western sky at sunset on July 9

6) On Wednesday the 9th, the Moon will reach First Quarter phase (sometimes called “Half Moon”), when it's 50% illuminated. It will be nearly midway in its eastward trip across the sky. Mars and Saturn will look as close as two cosmic peas in a pod. But they’ll appear their closest on Thursday the 10th, as Mars--which has a smaller, faster orbit than Saturn--speeds past the ringed planet.



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