Full moon border wars

Atlantic Sturgeon

Sturgeon are a large and primitive-looking fish native to lakes, rivers and coastlines. They have no scales and can grow up to 18 feet long. Their appearance isn’t just wishful thinking either. You can find fossils of early sturgeon species that are some 200 million years old. One of the best times to catch them is August, which is why Native American fishing tribes named the August full moon the Full Sturgeon Moon.

Tonight the moon will be nearly full and located in Aquarius the Water Carrier. Tomorrow it calls Capricornus home again. Created with Stellarium

We don’t know what they called the group of stars the Sturgeon Moon resides in, but to us it’s Capricornus the Sea Goat.  That’s where it will shine tomorrow night. Thanks to an odd bit of map making, the moon was in Sagittarius Thursday, skips over Capricornus and makes a brief appearance in Aquarius the Water Carrier tonight, and then returns to Capricornus tomorrow night. The moon and planets all move from west to east across the sky as they orbit the sun, spending time in one constellation before moving eastward into the next.

Constellation borders are shown in dashed lines. The moon (yellow dot) skips over Capricornus tonight but returns to the eastern side of the constellation tomorrow at full moon. Maps created with Chris Marriott's SkyMap

The strange leap from Sagittarius to Capricornus by the moon harkens back to how the 88 constellation boundaries were drawn. Originally the constellation boundaries were informal and consisted of casually-drawn snaking lines around the connect-the-dots mythological figures. As new objects were discovered, variable stars in particular, which are named after the constellation in which they appear, there arose the need to standardize constellation boundaries. That way astronomers in different parts of the world would know exactly which star they were talking about. Variable are stars whose light is not steady like the sun’s but changes over time due internal processes or eclipses by a companion star. If no one knew when one constellation ended and another began, how could they agree on a star name?

A wider view showing more constellations of the August sky and their well-defined borders.

In 1928 the constellation boundaries were formalized by the International Astronomical Union (IAU) after work done by Belgian astronomer Eugène Delporte. Delporte based his boundaries on vertical and horizontal lines called right ascension (R.A.) and declination Dec.). Similar to latitude and longitude, R.A. and Dec. fix a star’s position on the celestial sphere. As a result, our constellation borders resemble the squared-off borders of many U.S. states.

The boundaries were adopted after publication in 1930 and soon began to appear on star maps. Now you’ll find Delporte’s border work on everything from computer star charting programs to your mobile phone.

A final note: Don’t forget! Tonight’s the peak of the Perseid meteor shower. Although the bright, nearly full moon will blank out the fainter meteors, it’s still worth watching. Go out after 10 p.m. and face toward the east. The later you’re up the better, with the number of meteors increasing toward dawn.

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