9

While the eye is terrible at determining apparent magnitude or brightness of stars due to our adaptive iris, it's perfectly capable of discerning relative brightness between two objects in the same visual field. Also, the ancient Greeks could discern six magnitudes of star brightness by realizing that brighter stars look bigger in the night sky. The word ...


7

This is directly related to another question: Why are asteroids with zero orbital inclination rare? If captured, irregular moons are randomly oriented in space then there is very little chance of them having either inclination angles near zero or near $180^\circ$. This is because, if they are uniformly distributed in space, the fraction of orbits within a ...


5

There's a whole stream of them: https://iopscience.iop.org/article/10.3847/1538-4357/aadba3 Named Phlegethon, this is a stellar stream that covers about 75⁰ of the sky, and is probably the remnants of a disrupted globular cluster. It orbits the galaxy retrograde on orbits that take it between 4.9 and 19.8 kpc from the galactic centre. The orbits are highly ...


5

About two-thirds of the Milky Way’s estimated 100 billion stars reside in the disk, and these all move in direct orbits. Most of the remaining third are in the football-shaped central bulge, and while their orbits are more varied, on average they too are direct. A few percent of our galaxy’s stars occupy a vast, spherical halo. These travel on randomly ...


4

Variable star observers can obtain 0.1 magnitude accuracy by comparing the brightness of a star with 2 nearby stars of known magnitude; one a bit brighter and one a bit fainter. Most estimates are done telescopically with all 3 stars in view at the same time. The accuracy is much worse when doing this by naked eye and red stars are more difficult. So the ...


4

Mars's ~72 day retrograde loop occurs every ~2.1 years and intersects a given constellation once or twice every 15 or 17 years. The more information you have about the positions of other planets, e.g. Jupiter or Saturn, the more you can narrow the set of possible dates.


3

As Rob Jeffries says, no moons are in retrograde equatorial orbit in our solar system. One reason why prograde equatorial orbits are more likely than retrograde equatorial orbits has to do with tidal locking. Our Moon, for example, is tidal locked with the Earth. The Earth is actually also spinning down to tidal lock with the Moon. However, most ...


3

For a retrograde satellite, you are right that the satellite will migrate inwards towards the planet. Contrary to a prograde orbit, the rotation of the primary will slow down. Think about it in terms of angular momentum. Let the primary have a positive angular momentum and the satellite a negative one (since they rotate/orbit in opposite directions). Since ...


1

Go outside. Pick a star, any star and watch it for a while. It moves. As the Earth spins all the stars move across the sky, taking 23hr 56 min to move all around the sky to the same place. This is due to the rotation of the Earth (less than 24 hours due to the orbit of the Earth). We sometimes say the "celestial sphere" appears to rotate around ...


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