What is a blue moon?
A blue moon isn’t actually blue. The term is in fact used to describe the second of two full moons appearing in the same calendar month.
However, even if it isn’t an unusual colour, a full moon is still a majestic sight, and one surrounded with all manner of superstition and romantic implications.
The full moon is just one of eight phases during a lunar month however. During this cycle (lasting 29 and a half days) it changes constantly, moving from a fully lit disc to a completely invisible planetary body and back again. But what causes this cycle and how do we decipher the difference between a waxing gibbous moon from a waning crescent? It’s all a matter of perspective, and understanding this geometry is helped enormously with a simple clock analogy.
Let’s pretend that the Earth is at the very centre of a massive celestial clock face. From this central point the moon would sit on the hour hand travelling on its lunar orbit around the Earth. Constantly shining in towards the Earth from the three o’clock position is the Sun – it’s the Sun that lights the face of the moon so we can see it, except when it’s in the three o’clock position.
When the moon is right between the Sun and the Earth no sunlight touches the face of the moon that faces us. This is called a new moon and its apparent invisibility marks the beginning and the end of a lunar cycle. As the moon makes its way backwards – it travels anticlockwise around the Earth – from three o’clock towards half-past one a thin, waxing crescent moon grows in size as a small sliver of sunlight reaches a part of the moon seen from Earth. Once it reaches 12 o’clock the full right side of the moon is illuminated – this is the first quarter moon. When the moon reaches the half-past ten position the lit portion of the moon face is growing further still – this is known as a waxing gibbous moon.
At nine o’clock the moon is exactly opposite the Earth from the Sun meaning its entire face can be seen, but from this point on in the lunar month the moon is said to be waning from a gibbous moon, to a third quarter and then to a waning crescent before disappearing again at the end of its lunar cycle.
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