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Moon and star Regulus on March 18

On March 18, 2019 – the moon is moving through the constellation Leo the Lion, past Regulus, the brightest star in Leo, representing the Lion’s Heart. On these nights, the glare of the bright waxing gibbous moon might make it difficult to see all the Lion’s stars. But you’ll probably see Regulus, which is a 1st-magnitude star, that is, one of our sky’s brightest stars. And you might also glimpse the pattern of stars of which Regulus is a part. This backwards question mark pattern is a famous asterism within Leo, called The Sickle.

In another another week or two, the moon will drop out of the evening sky, allowing you to view the Lion in all his majestic splendor.

You’ll find the moon and Regulus on March 18 going westward across the sky throughout the night. They appear to move west for the same reason that the sun travels westward during the day, because Earth spins under the sky. The Earth spins from west-to-east on its rotational axis, causing the sun, moon and stars to travel westward each day and night.

From around the world, the moon and Regulus will climb to their high point for the night in late evening (before midnight). Afterwards, the moon and Regulus will slowly sink westward, to set in the west before dawn’s first light.

Chart of the constellation Leo via the IAU. The ecliptic depicts the annual pathway of the sun in front of the constellations of the zodiac. The sun passes in front of the constellation Leo each year from around August 10 to September 17, and has its yearly conjunction with the star Regulus on or near August 23.

For observational purposes, stargazers regard Regulus as a fixed star of the zodiac, even though this star appears to move across the sky throughout the night. Ancient astronomers, who were prone to thinking that Regulus and all the stars literally revolved around the Earth, still regarded Regulus and all the stars (excluding the sun) as fixed.

And that’s because the stars appear fixed relative to one another, much like dots in a connect-the-dots book. Modern astronomy has shown that the stars aren’t truly fixed; they’re actually moving, along with our sun, around the center of the Milky Way galaxy. Still, space is so vast that – in our lifetimes – we won’t notice stars moving relative to each other.

Meanwhile, objects in our own neighborhood of space, our solar system – sun, moon, stars, planets, dwarf planets, asteroids and comets – wander in front of these backdrop stars. We see them move because they’re so much closer to us.

The moon moves its own diameter eastward in front of the backdrop stars in about one hour. So – if you watch on the evening of March 18, and notice the distance between the moon and Regulus – then get up in the wee hours of March 19, you’ll see that the moon’s position has changed, relative to Regulus, the “fixed” star.

Bottom line: From North America, the moon will be pretty much due north of Regulus at nightfall March 22, 2019.

Bruce McClure

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