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.
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.
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.
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.
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.
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.