The Dark Side of the Moon

Our Moon is an odd ball. Very odd.

Pink Floyd had it right - there really is a dark side of the Moon. Let me explain.

Obviously Odd Features

Consider that the Moon always keeps the same side facing the Earth - we never see its back side from here. Scientists explain this as a feature of "tidal locking", where the internal stresses from slight bulging due to the forces of gravity have slowed the Moon's spin to a relative standstill. There are several other moons in the solar system that exhibit this strange behavior, so it isn't really all that odd, but still...

Our Moon is also very oddly sized when you consider that it perfectly blocks the Sun's disk during solar eclipses. If it were any smaller, or further away in its orbit, the Sun would only partially be blocked; any closer or larger and total solar eclipses would be commonplace. This is apparently just a rather bizarre coincidence, a feature of the Moon's size and distance being in exact proportion to the Sun's size and distance.

Made of Green Cheese?

Throughout history people have often been rather naïve about the Moon and how it works - everything from "it's made of green cheese" to simply perceiving a "man in the moon" face in its distant features. But people are curious creatures, so explaining how the Moon really works has been an ongoing challenge for millennia, with historically recent scientific results answering a lot of questions, and raising many more.

What Makes Them Roll Their Eyes

The "dark side of the Moon" has been a phrase used throughout the ages. Astronomers, scientists, and other smart people tend to roll their eyes when they hear this, and often you'll hear an explanation as to why there isn't really a dark side. Even though the Moon is tidally locked, it orbits the Earth once a month, causing the Sun to shine on all its parts. A "day" on the Moon lasts about a month, and the Sun shines brightly half that time for anyone up there, no matter which side of the Moon they are located on.

But Wait, There's More

But there's more to this story than just the Sun lighting up the Moon's surface. Imagine standing on the Moon, looking up at the night sky. If you're on the near side of the Moon, the Earth will loom over you, always and forever staying in the same spot in the sky. You'd see the Earth spin once every 24 hours, but more importantly, you'd notice the Earth is brightly lit up with month-long phases just like the phases of the Moon that we see from Earth.

Opposite Phases

Interestingly, the phases of the Earth and the phases of the Moon are exact opposites. That is, if the Moon appears as a thin crescent, then to an observer on the Moon at that same moment, the Earth would be almost completely full. Similarly, if the Moon is half lit, so is the Earth. But the Earth is much bigger in the Moon's sky than the Moon is in ours, so there's a lot of Earth light lighting up the Moon's night sky, especially when the Sun is "down", or located behind the Moon during its half-month-long night time. The takeaway from all this is that on the half of the Moon facing us, the sky is always lit up with either the Sun or the Earth.

My passion is to share Python code for everyone, and here's where I can go off on a short tangent to share a short program that tells you the phase of the Moon for any date between the years 1582 to 4000. Input a date and the program first calculates its Julian Day number, then uses that to calculate a reasonably accurate phase of the Moon. This script is presented in my book "Python for Numworks", but it runs great in any version of Python.

# moon.py
from math import *

def moon(m,d,y):
  j=jd(m,d,y) 
  n=(j+5.367)/29.53058
  x=n-int(n)
  p=int(abs(2*(x)-1)*100)
  w=int(2*x)
  return [p,w]

def jd(m,d,y):
  if m<3:
    y-=1
    m+=12
  a=int(y/100)
  b=2-a+int(a/4)
  e=int(365.25*(y+4716))
  f=int(30.6001*(m+1))
  return b+d+e+f-1524.5

m=int(input("Month (1-12): "))
d=int(input("Day (1-31): "))
y=int(input("Year (1582-4000): "))
phase,wax=moon(m,d,y) 
print("\nPercent lit: ",phase)
w="x" if wax else "n"
print("Wa{0}ing\n".format(w))

A companion program from the same book sketches the Moon phase. Here's what the Moon will look like from the Earth on July 4th, 2021. On this date the Earth will be lit an opposite amount, as seen from the Moon.

About that Dark side

Now consider the back side of the Moon, the side we can't see from Earth. Half of each month the Sun lights up that sky, a blinding beacon piercing the blackness of deep space. But for the other half of each Month, there is no Sun or Earth shine in the sky, only the vast expanse of stars and the Milky Way in an otherwise pitch black void. This is truly the dark side of the Moon, if only for half of each month.

Other Odd but Notable Features

In the not too distant future, people may honeymoon on the Moon, choosing either the truly dark side for deep sky watching, or the near side for the pleasures of seeing the phases of the Earth. It's easy to imagine special occasions, such as during lunar eclipses when the Earth would block the Sun entirely, all except for the highly anticipated, very bright, and very romantic "Ruby Ring of Earth" - the bright red sunset-like light visible in the thin layer of atmosphere around its edges. Of course the long corona streams, visible just before sunrise and just after sunset on the Moon, would be anticipated sights as well, although not nearly as rare of a treat.

Don't Keep Them in the Dark

So, next time someone tells you there's no dark side of the Moon, you now have the ability to explain to them there really is one. Except for those very bright stars up there, of course, but that's another story.

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