There are a lot of myths about the Moon; some are correct, and some are not, so we will debunk some incorrect myths about the Moon. The Moon is a big part of cultures worldwide and has been for thousands of years. It is a prominent fixture of the sky and has been the basis for many calendars. The Moon is fundamentally a large mass in space that is reasonably close to the Earth. Its motion is predictable. The incorect myths regarding the Moon link the Moon’s phase to Earth’s events, including human behaviour, fertility and agricultural practices.
We know that the Moon is responsible for some things, such as the tides and the stability of the Earth’s obliquity.
How bright is the Moon?
The Moon is at its maximum brightness when fully illuminated by the Sun. We call this the Full Moon. During a Full Moon, our dark sky meter from Star Safari Observatory reads 14.5 magnitudes per square arc second. During a New Moon, when the Moon’s illuminated side is pointing away from us, the reading can be as high as 21.87 magnitudes per square arc second. That tells us that when it’s a Full Moon, it is considerably brighter at night. Nothing too difficult so far.
The Moon is 3,474km in diameter. This reflects about 12% of the sunlight that hits it. As a science type, I like to have some evidence behind these figures. Saari and Shorthill (1972) did some measurements and came up with an albedo of 0.122, so the figure of 12% is pretty good to work with.
Let’s consider the Moon a big spherical mirror about 400,000km away. We calculate the flux of solar radiation hitting the Moon using the relationship between luminosity and the distance from the Sun to the Moon. We can assume it’s pretty similar to the amount hitting the Earth, which is 1365 Wm-2 (watts per square meter). The light reflected to Earth is only a fraction of this. If the Moon were a perfect mirror, we would get 0.025 Wm-2. The Moon is not a perfect mirror and only reflects 12% of the sunlight.
The light hitting the Earth from a Full Moon is approximately 0.003 Wm-2. This is about 450,000 times less than the light from the Sun.
How does that affect us?
That is plenty of light to wander around and not bang into things, but not enough for plants to grow. Research hasn’t found any strong relationship between Moon luminosity and plant growth. Research shows humans like to sleep in the dark, and we don’t like ambient light. The research into the effect of light pollution demonstrates the disrupted sleep patterns in inner cities. Maybe there is a link between the extra light during a Full Moon and human sleep patterns. As of yet, no significant link has been found. You can’t blame the Moon for your ratty moods when you wake up.
What about the Moon’s gravity?
The Moon is a big ball of rock 3,474km in diameter about 400,000km away. Something that big generates quite a bit of gravity.
Debunking some of the myths of the Moon challenges the belief that its gravitational influence varies with the phase. Some folks assume that when the Moon is full it has more gravity. That’s in the same vein as alchemy, astrology and homeopathy. It might have worked 400 years ago, but we know enough about the universe to debunk it now.
The orbit of the Moon is not circular and is inclined to the rotational plane of the Earth. This means that sometimes the Moon is a bit closer than other times. At its closest, the Moon is 363000km away, and at its maximum distance, it is 405000km. Newton worked out the gravitational force between two masses is inversely proportional to the square of the distance between them and proportional to the masses themselves. The gravitational force between the Earth and the Moon when the distance is 363000km is 2.22 x 1020N, and the force is 1.79 x 1020N when the Moon is 405000km away.
That seems to be a big difference?
That shows a nearly 25% increase in the force of gravity between the two objects when the Moon is at its closest. That’s not the whole story though. To understand the effect on us we have to think about it from the surface of the Earth. The acceleration due to gravity that we feel on the surface of the Earth is proportional to the mass of the Earth and inversely proportional to the square of the distance from the Earth’s centre. The radius of the Earth is 6.371 x 106m on average, and the mass is 5.9736 x 1024kg. That gives acceleration due to gravity on the Earth’s surface of 9.81ms-2. We’ll remember that value.
How much does the Moon contribute to this?
The Moon at perigee is 3.63 x 108m away, and the mass of the Moon is 7.349 x 1022kg. If the Moon were directly above our heads, that would reduce the Earth’s acceleration due to gravity to 9.80996ms-2 (so that is by 3.72 x 10-5ms-2). That’s the equivalent of 0.0004% – that’s hardly anything. When the Moon is 405000km away, the reduction to the Earth’s acceleration due to gravity experienced by us is 3 x 10-5ms-2. So a bit of a difference between the two distances, but completely lost in the overwhelming pull of the Earth.
In fact, the difference caused by the Moon is about the same difference in gravity you’d experience going up a mountain 1km.
If the Moon is so weak, how does it cause the tides?
The cause of the tides is all about the difference in gravity that Earth experiences from the Moon across its diameter. It only works because the Earth is so big. The pull from the gravity of the Moon is about 7% different from the side closest to the Moon to the other side, which is enough to cause a slight bulge in the Earth’s water towards the Moon. Out in the ocean, there is only a 1m tidal difference across a diameter of 12,000km.
Do the phases have anything to do with it?
No, nothing. The Full Moon doesn’t always match the perigee or the apogee. It’s almost the same gravitational influence at all phases. In fact, when there’s a Full Moon at apogee, the influence is ever so slightly less (0.0004% less). The phases only influence the amount of light the Moon reflects from the Sun to Earth.
If you want to learn more about the Moon, join us at Star Safari.
References
Saari, J & Shorthill, R 1972, ‘The Sunlit Lunar Surface’, Earth, Moon and Planets, vol. 5, pp. 161-178.
