THE SOUTHWORTH PLANETARIUM
70 Falmouth Street  Portland, Maine 04103
(207) 780-4249   www.usm.maine.edu/southworth-planetarium
43.67° N 70.27° W
Founded January 1970
2022-2023: CV
Sunrise: 5:00 a.m.
Sunset: 8:27 p.m.
Civil twilight begins: 4:24 a.m.
Civil twilight ends: 9:03 p.m.
Sun's host constellation: Gemini the Twins
Lunar phase: Waxing crescent (18% illuminated)
Moon rise: 8:53 a.m.
Moon set: 11:46 p.m.
Julian date: 2460118.29
     'Keeping a watchful eye on a complex sky"

THE DAILY ASTRONOMER
Thursday, June 22, 2023
On the Perpetual Half Moon

On today's Pandora parchment the following question was inscribed:
'*If the moon is always half lit up, why does its apparent shape constantly
change?'  -B.P., Telford, UK*

We observe lunar phases because the moon, which is always 50% illuminated,
revolves around us, altering our perceptions of it.

However, let’s suspend physical laws and pretend that the moon maintains
its same position in its orbit. For instance, let’s place the moon at
Earth’s far side relative to the sun:

During this phase, called opposition, the entire illuminated half of the
moon is directed toward Earth. Consequently, the lunar disk appears wholly
illuminated:

We see a full moon. If the moon stood still, each night we’d

see a full moon.

Now, let’s shift the moon’s position so that it occupies a fixed position
between the moon and Sun.

If the moon maintained this position, its illuminated half would always be
directed away from Earth and would always remain in the phase called ‘new
moon’ or conjunction.

The moon doesn’t remain in the same position, but instead revolves around
Earth continually. As a result, we see varying amounts of the moon’s
illuminated half.

The percentages are:

   - 0% conjunction “New moon”
   - 50% first quadrature “First quarter moon”
   - 100% opposition “Full moon”
   - 50% second quadrature “Third quarter moon”

The phases occur simply because the moon remains half illuminated while it
describes an orbit around our planet.
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