Already this I can see this is going to be harder than I thought, and it’s only my first post! Probably because I already know what I’m seeing in this particular case, so maybe later posts where I’m learning new stuff will be easier.
Over the last few months we had clear skies in the evenings. Each day the sky looked still, although the stars gradually moved from one part of the sky to the other. But relative to each other, all the stars were in the same place and didn’t move.
But there were two other stars that did move relative to the others.
I’ve seen these two stars before, and always roughly in the same region in the sky at the same time of the evening, around the same time of year (Spring). There was one very bright one, and one less bright one. Both always appear in the sky as the Sun is setting, and are in the same region of the sky as the Sun.
The fainter star was quite bright at first, but then at the same time each evening, when the Sun was just below the horizon, I could see it getting fainter and fainter as it got lost in the glare. It also appeared to be moving closer to the Sun.
The brighter star was much easier to see, and as the days and weeks went on (same time each evening) the star seemed to be moving away from the Sun. Indeed, it got so far away from the Sun that when it was very dark and the Sun was far below the horizon, it could still be seen. It got brighter the further away it moved. But eventually, it began to move closer to the Sun until eventually it too was engulfed in its glare.
This pattern repeats itself in a not too unsimilar way each time I observe these unusual stars. So, as the vast majority of stars are stationary in the sky, I deduce that these are something different.
With this observation earlier this year I discovered the planets Venus (the bright one) and Mercury (the fainter one). These planets, as well as Mars, Jupiter, and Saturn, were all known from ancient times, as they all moved differently to the other stars in the sky. They were given the name planet, derived from Greek meaning ‘wanderer’.
Venus and Mercury orbit the Sun closer than Earth. This means that we always see them somewhere between us and the Sun, and this explains why they’re always found in the same region of the sky as the Sun. The brightness change is a combination of both distance from the glare of the Sun, but also phase change, as the planet travels around the star, much like the Moon has Full Moon, New Moon, and Quarter Moon phases.