The Rules to Being a Moon

We all know the moon that orbits around us, it is the only natural satellite of Earth. The moon also does more than light up the night light, the moon makes earth a more liveable planet by moderating any fluctuation that occurs in earth’s axis. The moons gravitational pull generates something called the tidal force. The tidal force results in Earth and it’s water to bulge out on the side closest and farthest to the moon, and these bulges of water are called high tides.

Figure 1 – Jones, Graham, et al. “What Causes Tides?” time and date, www.timeanddate.com/astronomy/moon/tides.html.

But the real question how we classify moons and the difference it has in contrast to space rocks. The definition of a moon is defined to be a “celestial body that makes an orbit around a planet, including the eight major planets, dwarf planets and minor planets” as stated by an article found on Universe Today. Space rocks and other satellites of come in a range of size, shape and their pattern of orbit each of these factors being a contribution to be considered a moon.

As our space technology advances and we are able to spot smaller and smaller objects in space and have identified 60 increasingly small moons orbiting Saturn alone, some even as small as a kilometer-wide rock within Saturn’s ring. For a natural satellite is considered a moon, it needs to have checked off these given conditions.

Figure 2 – Lanphier, Mary. “Since the Sun and Moon move from east to west, why did the eclipse move from west to east?” Astronomy, edited by Alison Klesman, Astronomy, 28 Feb. 2018, astronomy.com/magazine/ask-astro/2018/02/eclipse-direction.

A moon must be of such size that the common barycenter lies entirely within the parent planet

A moon must be in a stable orbit around a planet, dwarf or minor planet

As mentioned, before we can see space with a new lens thanks to improvements in technology and as a result, we can see space in greater detail. If we make the size requirement of a moon too little, we will be essentially giving the tittle to billions of rocks and dust particles inside the rings of Saturn moons? Now that we understand the minimum limits, how can we tell when a moon is too big to be considered a moon?

Figure 3 – The Barycenter of the Solar System, The Earth Chronical of Life, 3 Nov. 2016, earth-chronicles.com/space/the-barycentre-of-the-solar-system.html.

Believe it or not, when something orbits another body of mass, they are both orbiting their common center of mass, which is called the barycenter. Where that point lies is varied from the masses of the two or more objects that are in orbit.  For reference it feels right to say that our moon orbits us as the barycenter is understandably well within earth. An example where the moon it too big compared to the parent planet is Pluto and Sheron. As shown in the gif, their masses are contrasting they are similar enough that the barycenter is located outside of Pluto.

Figure 4: Retrieved from Wikipedia, 12 July 2021, en.wikipedia.org/wiki/Barycentric_coordinates_(astronomy).