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Sunday, 12 June 2022

Some astronomy lingo!

A couple of people have contacted me about the various terms used in lunar astronomy, so here are a few frequently used ones. It's worth recalling that the phases of the Moon are produced by the changing angle between the Sun and Moon as it rotates anticlockwise around the Earth. When the Moon is directly opposite the Sun we see a 'full Moon': when it is on the same side, the 'New Moon' is invisible (unless it happens to pass in front of the Sun, resulting in a solar eclipse.)

Last night there was a beautiful waxing gibbous Moon:

waxing (from old English 'wachsen', to grow) As the Moon moves on from 'new' its visible area grows larger

waning (from old English 'wanian') to make or become smaller

gibbous (from latin, 'gibbus', a hump) When the Moon's illuminated region is larger than half its disc 

first / last quarter. The Moon is, of course, a sphere: when we see a 'half Moon', we are actually observing a quarter of its surface. As the Moon's phase waxes from New Moon, it reaches first quarter: as it wanes from full Moon it shrinks to last quarter.

One day past New Moon

First quarter

Last night's waxing gibbous 

Full Moon

Waning gibbous

Last quarter

One day before New Moon

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