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We’re back for part two of the 100 subscriber Q&A special, so let’s dive right in. The first question is “What makes an orbit eccentric?”. This is a great question, mostly because eccentricity is such a complex thing. Many of the planets in our solar system a lot of the orbits are minimally eccentric compared to comets or many exoplanets, and how eccentric they are can vary over time. The variation in eccentricity over time seems to be the influence of other planets, tugging on the planet as it orbits the Sun. Moons can also vary in eccentricity, and some moons are more, or less eccentric than others. Neptune has a particularly non-eccentric moon going around it, the moon Triton. Triton is a huge moon with a practically circular orbit. If we take a look further out to the Oort Cloud or the Kuiper Belt, a lot of things out there orbit pretty uniformly. However, this is also where many comets originate, and their orbits can be highly eccentric. It is thought to be the action of the planets in our solar system pulling on those distant objects, that disturbs their orbit, causing them to start falling in towards the Sun. It may also be passing stars or the as of yet undiscovered Planet X that changes their orbit from almost circular to highly eccentric. A lot of eccentricity may be explained by the interaction between different objects, but now that we;ve discovered other solar systems, we are starting to see new questions.

I mentioned in the previous video, which tackled some of the earlier questions in the Q&A, that our solar system doesn’t appear typical compared to the others that we’ve discovered. The relative lack of eccentricity is one difference. Generally, unless a planet is stuck with one face towards its star, in a circular and stable tidally locked orbit, then it will have a rather eccentric orbit, even if it is a massive gassy planet. There is a good chance that when the cloud dust and rock, the protoplanetary disk, around a small star first starts coalescing together to form planets, that eccentricity becomes a part of these early planet’s orbits. As the planetesimals, the little balls rock that eventually smushed together to form full-blown planets collide together, they fuse, turning two orbits into one new one. That, or they end up stuck as dwarf planets, like Ceres in the Asteroid Belt may be. These collisions may cause the fledgling solar system to begin with a lot of eccentricity. In the video we taka a look into the constellation of Orion, where protoplanetary disks have been discovered. These young solar systems, or proplyds, give us a chance to see how the early solar system may have looked. If these early solar systems did in fact begin with a lot of eccentricity, the interaction of different objects within their orbit around the star would still seem to be the biggest cause of that eccentricity. That highly eccentric state may be the default, but we’ve only discovered so many solar systems, a small fraction of the potential amount of solar systems that could be out there, so we don’t have a big enough sample size to say for sure so far. However, it does look like the circular stable orbits are in our solar system might be the exception, rather than the rule.

So that leaves us with the question of what caused our solar systems lack of eccentricity. There is a theory that our solar system formed looking very different to the way we see it today, and more like what we see in other solar systems. As we see it today, we’ve got our big planets like Jupiter and Saturn and then further out Uranus and Neptune and then close in to the center we have our rocky planets. Around a lot of other stars, a lot of exo-solar systems, we see massive gassy planets quite close to their parent star. There is a chance that that’s where our Jupiter and Saturn formed, or at least our Jupiter. If Jupiter formed much closer to the Sun, it would have needed to migrate to the orbit we see it in today, crossing the orbits of other planets to do so. This is the Great Tack or Grand Tack Theory, that Jupiter tacked its way across the solar system like a sailboat, and forced the inner planets into a more circular orbits as it went. This theory has the double whammy of explaining both why our orbits are strangely circular and why our gas giants are uncommonly far out from the star. Then again, this is based on only what we have discovered so far, and without time-travel we may never know. So besides eccentricity being caused by the interaction of different objects, the lack of eccentricity, this “circularization” of the orbits in our solar system, may have also been caused by interactions between the massive of planets and the smaller planets closer to the Sun. I think this makes eccentricity a very interesting question.

Next up we are going to take a look at the Moon. We take a look at the Moon a lot here and for good reason, it is pretty much the most prominent thing in our sky. It’s one of the only things that we see almost every night, only on the New Moon and the nights close to the New Moon will the Moon be completely missing. We do always see the same side of the Moon, the same face, but that doesn’t mean we always see exactly the same parts of the Moon. Over the course of months the Moon wobble sa little bit as it orbits around us. The Moon appears to roll or tilt “backwards” a little bit, allowing us to see a little more of the South Pole of Moon. The it rocks forward a little, letting us see more of the Northern portion. This rocking or rolling of the Moon is called Lunar Libration. This let’s us see more than 50% of the Moon, people say that we see half the Moon and the other half is facing away from us, but really we get to see about 65% of the Moons surface over the course of the year, and the rest is always facing away from us. This is one surefire way to see that the Moon is a ball and not a disc. Of course, taking a close look at the moon when it is a little before a half, either side of the FIrst Quarter or the Last Quarter, gives us an opportunity as well. We can see the curved shape the shadow, which hints at the curved nature of the Moons surface. Over longer periods of time we get to see this rolling, which shows us that there is more of the Moon than the flat surface it can appear to be if you just look at it on one night. That’s not the only way we know that the Moon is round of course, we have sent things to the Moon, we’ve sent astronauts to the Moon, right in to the Sea of Tranquility for Apollo 11, the other Apollo missions landed in other areas. We haven’t all been lucky enough to orbit or walk on the Moon, but you can still make sure that the Moon is spherical yourself, just by looking at it with your own eyes or using your own homemade telescope.

That is something you can do and it is something you can do because the Moon is spherical, you can check and see that it is spherical because it is. The same is true for the Earth, and I bring this up due to the phrasing of the question, “why do people think the Moon is round when the Earth is flat?”. I’m glad that the question was asked that way, because it starts the question with an assumption, it assumes that the Earth. This is a form of logical fallacy, assuming your conclusion in the question, it’s almost like saying, “Well, if Earth is flat, how is the Moon round?”. A question without assumption is often better, we can check if the Moon is round without anything to do with the shape of the Earth. Instead of assuming that the Earth is flat, we need to challenge that assumption. You can ask “What shape is the Earth?”, and of course there’s many ways to do that. By setting up a big stick in two different locations, two different latitudes most importantly, and looking at where the shadow falls at midday, you’ll see that the shadows fall in two slightly different locations. One stick will have a shorter shadow than the other because of the angle at which the Sun is shining at the Earth. This can help show that the Sun hits different sticks at a different angle, as the surface of the Earth curves away from the Sun t higher latitudes. You can also watch things drift over the horizon, especially if you are at the coast and near a harbour. You could also take a trip to a tall mountain and see how your view increases as you are able to see further around the curve from your higher elevation. In some tall buildings, you can watch the Sun set on the ground floor, and then take an elevator up to the top to see another sunset! This is because you’ve moved up to a point where you can see further over the horizon, or further beyond the curve of the Earth, bringing the Sun back into view. Just like you can visibly see that the Moon is spherical under the right conditions, you can also visibly see that the Earth is spherical as well.

The final question is a good excuse to bring up the constellations, not that I need one, and I show the constellations quite a lot. I was asked the question “Given that constellations mostly come from Greek and Roman mythology, are there constellations from other mythologies?”, which is another great question. The official 88 constellations that we use in modern astronomy defined by the International Astronomical Union, these do only come from Greek and Roman myths or more modern explorers, but they are not the only ones to have existed. The official constellations might only draw from a pretty small set of the world’s cultures, but just as there is and has been a great variety in human culture around the world, there has been a great variety in constellations as well. Of course, other cultures will generally name the stars themselves in their own language, at least the prominent or important ones. Many cultures, including Indian Vedic and Japanese traditional astronomy, paid great attention to the Moon and planets travelling across the sky. This of course led to a similar set of regions in the sky to the ecliptic and the zodiacal constellations in Western astronomy. In Ancient Chinese astronomy, the North Star and northern Portion of the sky were the focus, being considered a region or enclosure unto itself.

Of course, these different cultures also joined together different stars. Many constellations in other cultures would combine or overlap the ones we normally use. Then again, the brightest stars are the brightest stars no matter where you are, and certain shapes tend to stand out. The Inuit culture had its own conctellations, and one of them uses most of the stars in our Ursa Major, but rather than having a bear, or the Plough and Big Dipper, it is a reindeer or a caribou (whichever name you use for the species). You can also see in the video that the brightest star in the sky is named called Flickering. This is the same star that we would call Sirius in Western Astronomy, and it is given its own name in Inuit astronomy as well, presumably because it’s the brightest in the sky, and it it’s the brightest in the sky no matter which culture you come from. The Ojibwe constellations given in Stellarium are particularly nice, showing a very different style of art. Just like the mythical creatures used in our constellations, we can see creatures like the Thunderbird and other creatures from Ojibwe mythology. It is also clear that they are still constellations, highlighting very specific stars at different parts of the picture. We can also see shapes based off the collections of stars that they see, the same bright groups we have our constellations based on, but of course they are very different to the ones that we use in western astronomy. This is simply because it was a whole other culture that came up with them, on their own. These Ojibwe constellations, come from a time before any contact with the Europeans and their Greek and Roman constellations. The same is true for constellations coming from other parts of the world. As well as different words for things like stars and planets, and different names for them, different stars get joined to make different pictures, depending on what’s prominent in the cultures mythology. In short, yes there are constellations that come from other cultures, but they exist in their own context, their own group of pictures in the sky. They’re not usually mixed in with the constellations that we generally talk about in modern astronomy, the official 88. Although other cultures, such as Native American, and things important to other cultures, like Uluru (formerly known as Ayers Rock) do feature in our official constellations, thhose images were still conceived by Western explorers, coming from the same culture that made the rest official. Pretty much every culture is believed to have had their own set of constellations, but unfortunately not all of them have survived in detail to the modern day, and for many we may never know them.

I do briefly mention at the end of the video that Mars is starting to appear at sunrise, it will get easier to see as we push further into the year. Otherwise, that is all the questions answered, all the ones I was asked for the special Q&A for celebrating 100 subscribers. If you would like to ask questions go right ahead, here, on YouTube or on Instagram. I don’t know when my next special video will be but there will be another one in the future, I have no plans of stopping this anytime soon. I hope some of your questions got answered here as well, or maybe some new ones have come up, either way I hope to see you here again next time.