Pomona moved the celestial models Albus had once used as decorations in the Defense Against the Dark Arts classroom into the Astronomy classroom.
These things could still be used after a little cleaning. There was no need to buy new ones. If they bought everything on the Astronomy Club's list, the budget would rocket into the sky.
Humanity had not yet left the Solar System. The farthest anyone had gone was the Moon, yet their sight already reached dozens of light-years away.
As Pomona wiped the model of the Sun with a magical cloth, she thought that variable stars were also a kind of star. It was simply that the brightness of most stars was almost fixed. Regardless of a star's distance, stars as seen from Earth were divided into six magnitudes according to their apparent brightness: first-magnitude stars were the brightest, and sixth-magnitude stars the dimmest. In truth, some stars that appeared very bright were not especially luminous; they were merely close to Earth.
Take Sirius in Canis Major, for example. It was one of the stars closest to Earth. Then there was Alpha Centauri, which could be called the Solar System's Proxima Centauri. That system consisted of three stars.
Newton's law of universal gravitation predicted the motion of two celestial bodies attracting each other. Their orbits were basically elliptical. But what happened when there were three bodies, such as the Sun, Earth, and Moon? What laws governed their orbital motion?
That was the famous three-body problem. Lagrange points were a special solution to the three-body problem. At such a point, a small asteroid remained basically stationary relative to two larger celestial bodies.
If there were an artificial Earth satellite, it could remain at a Lagrange point between the Sun and Earth without falling into the Moon's gravitational range.