May 25, 2024

Starwatch chart

This week the moon reaches its first quarter phase, and in celebration sails past the star Regulus in the constellation Leo, the lion. Regulus is part of the Northern Hemisphere’s Spring Triangle (see last week’s Starwatch) and is therefore well placed in the southern sky this week.

The chart shows the view looking south at 22.00BST on 15 May 2024. At the time of conjunction the moon will be 7.9 days old, so will have just crept past the exact moment of first quarter. Instead of being a waxing crescent, it will be a waxing moon with 55.7% of its visible surface illuminated. Regulus is about four times more massive than the Sun, and has a surface temperature of 11,500 K – roughly double our own Sun’s 5,700 K.

Both stars are in the stable “middle-aged” sections of their lives. This is known as the main sequence and involves the stars generating power by converting hydrogen to helium through nuclear fusion reactions. The conjunction is also easily seen from the southern hemisphere, although depending on the observer’s longitude, the moon may not be quite as close to Regulus during the hours of darkness.

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