Exploring the waxing gibbous moon

Image: Ian Griffin
Image: Ian Griffin
The phrase "waxing gibbous moon" originates from the Latin roots "waxing", meaning to increase or grow, and "gibbous," referring to a shape that is convex or bulging. This term describes the phase of the moon when it is between the first quarter and full moon, appearing more than half but not fully illuminated by the sun from our perspective on Earth. With the moon reaching first quarter this morning, it will be waxing gibbous for the rest of the week.

I love observing the nightly changes of the waxing gibbous moon. Each night, as the sun rises over the lunar landscape, a new set of craters and seas is revealed. I’m particularly fond of watching the sun rise over the Copernicus, a prominent impact crater in the Oceanus Procellarum, the Ocean of Storms.

While scanning the moon with binoculars or, better still, a telescope is always interesting, there’s another excellent reason to head out after sunset on Monday. That’s because shortly after 9.40pm, the 77% illuminated moon will pass before a relatively bright star called Kappa Aurigae. This is interesting for several reasons.

Firstly, watching a star vanish behind the moon is always fun. If you turn your telescope moon-ward shortly after 9.30pm on Monday, you should spot Kappa Auriga just to the right of the dark lunar limb. Kappa is an orange giant star located nearly 180 light years from Earth. Over the next 10 minutes, the moon will slowly approach the star, and then suddenly, at approximately 9.45 pm the star will "blink out". Astronomers call this event a lunar occultation.

Secondly, this occultation is interesting because the covered star isn’t in one of the 12 traditional zodiacal constellations. The moon is passing through the outskirts of the constellation Auriga, the charioteer, named to honour Erichthonius, King of Athens.

Erichthonius was the first person to harness four horses to a chariot, inspired by the four-horse chariot of the sun. This remarkable feat impressed Zeus so much that he granted Erichthonius a place in the celestial realm.

Fingers crossed for clear skies on Monday.