The Universe And All That
Is the Moon A Planet Or A Star?
That question was posed in a video on our MAS Facebook page recently by a pair of nauseating astro-illiterates, “debating” on a ladies fashion TV show, the two nonsensical alternatives.
Her: “I believe it’s a star or something.”
Him: The Moon is a planet, Honey.
The status of the Moon should be understood by all but I guess they would probably both roll their eyes over my lack of basic ladies fashion knowledge. We should know from our own outreach events that while some people are clued up, many others in the population don’t have any idea about astronomy. It’s up to us to try and remedy that.
Of course we all know that the Moon is a natural satellite and is neither a star nor a planet. But I want to look at this in a bit more depth. So let’s rule out the star “theory” first.
Stars predominately consist of hydrogen and other light elements. They emit vast quantities of light and heat due to thermonuclear fusion reactions in the core. The Sun is a star. We feel its warmth but the Moon only radiates weakly reflected sunlight. Sometimes the Moon doesn’t shine at all. That should have been a big clue to the lady who was adamant that the Moon is a star.
It’s not too hard to establish that the Moon is not a star but why isn’t it a planet like the guy said? It’s smaller than Earth but it’s still pretty big and it looks like a planet.
Defining The Solar System
In 2006, the International Astronomical Union (IAU) mandated three conditions for a Solar System body to be classified as a planet. Remember? The big loser was Pluto!
The IAU defined a planet as follows:
A “planet” is a celestial body that
- (a) is in orbit around the Sun,
- (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
- (c) has cleared the neighbourhood around its orbit.
Pluto ticked two boxes but failed (c) because it hasn’t cleared its orbit of debris.
The Moon cannot be defined as a planet, because it failed to tick box (a) (because it orbits a planet, not the Sun). It ticked box (b) and arguably ticked box (c). The Moon did its share of orbit clearing. Just look at the number of lunar craters!
Size Doesn’t Matter
It’s the ‘definition’ which calls all the shots – not size or appearance – and our fashion-loving friend, who believes that the Moon is a planet, would have been considered correct, had he lived in earlier times, when for centuries the geocentric model of the Solar System was both mainstream science and dominant religious dogma. This belief stated that the Sun and planets all orbit the Earth. The Moon was considered a planet.
Both Copernicus and the IAU changed the definitions of bodies in the Solar System.
When the Copernican heliocentric Solar System model was finally acknowledged, the Moon was “demoted” to satellite status, a similar fate to that which Pluto later suffered in 2006 when it was “demoted” to dwarf planet status.
It’s interesting that Pluto is smaller than seven natural satellites, including the Moon, so the 2006 redefinition of the ex-planet was probably justified. Two of those satellites, Ganymede and Titan, have a diameter larger than the planet Mercury.
Notwithstanding the IAU official definition, which of course I totally accept, the Moon/Earth System has the largest diameter ratio of satellite to planet (0.27) of all, so in my mind the Earth-Moon system could very easily be described as a double planet system.
But that doesn’t make that fashion shop guy right.
|This article first appeared in:|
Prime Focus Magazine, April 2020.