Old moon, Regulus, rising times and more

These upcoming mornings – September 26 and 27, 2019 – the waning crescent moon and the star Regulus adorn the eastern sky before sunrise. Astronomers will call the waning crescent, visible at dawn, an old moon. Regulus is a bright star and the brightest light in the constellation Leo the Lion. You might or might not be able to see in your sky that Regulus marks the bottom of a backwards question mark pattern of stars. This asterism is called The Sickle, and it marks the head and shoulders of Leo. Regulus is sometimes called The Heart of the Lion. The name Regulus means little king.

Illustration of constellation Leo, with The Sickle marked.

Regulus is part of a backwards question mark pattern known as The Sickle in Leo. Image via Derekscope.

Now … about the time of sunrise. Because we’re only a few days past the equinox, sunrise happens about six hours before noontime – or about six hours after midnight – everywhere around the world. In this usage, noon means midway between sunrise and sunset, and midnight means midway between sunset and sunrise.

On the other hand, Regulus is a northern star, so it rises at an earlier hour at more northerly latitudes and a later hour at more southerly latitudes. For the next day or two, we give the approximate amount of time that Regulus rises before sunrise for various latitudes:

60 degrees north latitude: Regulus rises 3 1/2 hours before the sun

35 degrees north latitude: Regulus rises 2 1/2 hours before the sun

Equator (0 degrees latitude): Regulus rises 2 hours before the sun

35 degrees south latitude: Regulus rises 1 1/3 hours before the sun

60 degrees south latitude: Regulus rises 1/2 hour before the sun

For your specific view of Regulus before sunrise, try Stellarium Online.

The moon is a harder to pin down, because it’s not a “fixed” star like Regulus. In fact, during the one day (24-hour) stretch from September 26 to 27, the waning crescent moon moves 15 degrees (30 moon-diameters) closer to the sunrise point on the horizon for everyone worldwide. Yet, the rising time of the moon depends on your latitude – and your longitude.

Earthshine on old moon.

Here’s an old moon, caught by Amirul Syazani in Port Dickson, Malaysia in 2017. That glow on the darkened portion of the moon is called earthshine. It’s twice-reflected sunlight – sunlight reflected from the Earth to the moon, and then from the moon back to Earth. Watch for it on the moon in the coming mornings.

For instance, sky chart at the very top of this post – showing the moon and Regulus – is especially for North America. Nonetheless, the moon will be in the vicinity of Regulus on the sky’s dome as seen from around the world. At the same date from the world’s Eastern Hemisphere, at one hour before sunrise, the moon appears more westward (upward) relative to Regulus than it does from North America; yet from the Hawaiian Islands, the moon appears more eastward (downward) than it does in North America.

However, from everywhere worldwide, the moon will be harder to catch on September 27 than on September 26. That’s because the shrinking crescent looms closer horizon and is more deeply buried in the glow of morning twilight.

Old waning crescent moon with Regulus at dawn.

View larger at EarthSky Community Photos. James Trezza caught the star Regulus to the right of the waning crescent moon before sunrise September 26, 2019, from Long Island, New York, USA. Thank you James!

And here’s another factor that’ll determine how you’ll see the old moon and Regulus in the coming mornings. The waning crescent moon will be easier to spot from the Northern Hemisphere than from the Southern Hemisphere. That’s because the ecliptic – approximate monthly path of the moon in front of the constellations of the zodiac – hits the sunrise horizon at its steepest angle for the year around the time of the autumn equinox, which we just passed. Meanwhile, at the spring equinox, the ecliptic intersects the sunrise horizon at its shallowest angle of the year; that’s the case in the Southern Hemisphere now.

So – as the moon gets closer to new – the advantage for any old moon hunt goes to the Northern Hemisphere. The moon is now a bit north of the ecliptic, which adds to the Northern Hemisphere’s advantage and to the Southern Hemisphere’s disadvantage. Many factors to consider! And yet we’ll all see the old moon and Regulus before sunup in the coming days.

New moon will fall on September 28, 2019, at 18:26 UTC. This new moon will be a supermoon – an extra-close new moon.

Some two weeks before this upcoming new moon, the September 2019 full moon presented the farthest full moon of the year. Depending upon your time zone, this full moon fell on September 13 or 14, 2019. If, somehow, we could superimpose this September 2019 full moon (mini-moon) onto the September 2019 old crescent moon, the size difference would resemble Peter’s magnificent portrayal of a lunar crescent at perigee with the full moon at apogee in the photo below.

Peter Lowenstein superimposes the small June 2017 mini-moon (full moon at apogee, farthest from Earth for the month) with the slender lunar crescent, with its dark side covered over in earthshine, when at perigee (closest point to Earth). The most recent full moon on September 14, 2019, was a mini-moon, and two weeks later, the old moon will sweep to perigee (closest point to Earth) on September 28, 2019, at 2:27 UTC. The moon reaches perigee only 16 hours before the moon turns new on September 28, 2019, at 18:26 UTC. If, somehow, we could superimpose this September 2019 mini-moon onto the upcoming old crescent moon, it’d probably resemble Peter’s magnificent portrayal of a supermoon with a mini-moon. The size difference is proportionally comparable to that of a U.S. quarter versus a U.S. nickel.

Bottom line: Enjoy an early morning drama as the waning crescent moon swings past the star Regulus on the mornings of September 26 and 27, 2019.

Bruce McClure