Half Moon Has Close Brush With Spica Tonight; Aurora Update

The moon and Spica at nightfall Monday July 15. Maps created with Stellarium

The jaunty moon bounds eastward about one outstretched fist a day as it orbits Earth. Tonight it will pass by Spica and Saturn, that pair of bright “starry eyes” in the southwestern sky at dusk’s end.

The moon slides by Spica on the evening of July 15. This panel shows the view from the northern U.S. where the two will be separated by about 2/3 the moon’s diameter. It should make for a great naked eye and binocular sight.

As seen from most of North America, the moon, now filled out to a half, will pass immediately to the south of Spica. It’ll be fun to check in on the moon’s position in relation to Spica every hour or so; you’ll see it move to the left like a hand on a clock.

The farther south you are the closer the two will be. From southern Mexico, Central America, the Galapagos Islands and western Pacific, the dark encroaching limb of the moon will cover or occult the star completely for up to an hour. Click HERE to get times when the occultation happens for your city. Don’t forget to subtract 6 hours from the times shown to convert to Central Standard.

My older daughter Katherine demonstrates how to see parallax by staring at your outstretched thumb while alternately opening and closing each eye. Credit: Bob King

You might wonder why the moon misses Spica for U.S. observers but covers it farther south. It has to do with parallax, the apparent shift in the position of a foreground object like the moon against a distant backdrop like the stars depending on your viewing location.

You can see parallax at work by sticking an arm straight out in front of you and doing a “thumbs up”.  Open and close your right and then your left eye in a back and forth blinking pattern. As you do, your thumb will appear to jump back and forth across the more distant background. Each eye sees the thumb from a slightly different perspective, causing it to shift position against the distant scene.

If you know the distance between your eyes – called the baseline – and measure the angle made by your dancing thumb, you can use basic trigonometry to find how many inches your thumb is from your face. Astronomers apply the same concept to measure star distances using the diameter of Earth’s orbit, the equivalent of two eyes separated by 180 million miles. The longer the baseline, the farther you can measure.

The moon’s position against the distant background stars shifts depending on your viewing location. From points south, the moon is a bit further north in the sky; from points north, it’s shifted to the south. Illustration: Bob King

Now you understand why the moon appears to shift position in the sky depending upon where you are on Earth – parallax! The shift amounts to about one degree or twice the diameter of the full moon. That’s if you could simultaneously observe the moon from two locations at either end of the planet, say from the north and south poles which are 8,000 miles apart.

In March 1988 the crescent moon passed near the Pleiades. In this illustration you can see its position in relation to the cluster from four extreme locations. Credit: Tom Ruen

Closer to home, the moon’s shift is only 1/4 of that for observers located in International Falls, Minn. and Miami, Fla. 2,000 miles (3,200 km) and not even noticeable between Duluth and Minneapolis.

Aurora panorama taken on July 13-14 from Reesor Ranch in southwest Saskatchewan by Alan Dyer. Click to see more of Alan’s excellent photography.

There’s a lot going on up there. Take a look tonight if the sky’s clear by you. That brings me to the aurora. The weekend’s hoped-for display of northern lights finally materialized last night (some activity on July 13/14 too) and continued through the early morning hours. We were cloudy in Duluth, Minn. but skywatchers from Washington to Wisconsin to New Zealand caught sight of them. There’s a small chance for more this evening as the display subsides.


5 Responses

  1. Phil

    Is there a site that estimates the time or is it more of a guessing game? Just a newbie trying to figure out photography. Thanks for the great info!

  2. Phil

    Sorry. The best time to see aurora. Reading your posts it sounds as if it can happen at any time of the night or early morning.

    1. astrobob

      The aurora can happen at any time of course. It might be going crazy over northern Europe when it’s afternoon in the U.S. Then by the time it’s dark here, it’s over. Generally, if aurora is in the forecast, the best time to see it is around local midnight (1 a.m. if you’re on daylight saving time). Early this evening, all’s quiet, but I notice a slight southward dip in the direction of the IMF (solar magnetic field present in the Earth’s vicinity). If that dip continues, the aurora could fire up again like it did early this morning.

  3. Edward M. Boll

    I had my hand over the Moon to see Spica, incredibly beautiful within the lunar glare. In late August when ISON hopefully comes into view, Siding Spring A1 reaches more than 75 degrees from the Sun. This comet has a poor placing for northern observers at least through the first half of 2014.

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