Lately, I’ve been joined by a familiar sight every evening on my way home. The planet Venus sits there in the evening sky, shining like a distant beacon. Once you take notice of it, it’s hard to not see it there night after night. It’s really fascinating.
Earlier tonight, as I was walking to the station I paused to think: why does it seem like I’ve only noticed it as of late? For that matter, why is Venus given both “evening star” and “morning star” epithets? And what’s this about Venus going into retrograde motion? It’s actually all kind of related. There are, several concepts here that you might not consider together, but when you do you can’t help but get some greater appreciation between nature and geometry of it all.
(at least, that’s how I feel)
So, all right, let’s lay some foundation here. All of the stuff I’m explaining here is in my own words and might not be totally accurate, but I’ll try to be as correct as possible.
From Earth, all of the planets, and the sun and moon appear to move across a band of sky called the ecliptic. This band is where the constellations of the zodiac are located. Anyway, as the day goes the part of the ecliptic that we see keeps changing due to the rotation of the earth. Just like the sun, all of the bodies just rise and fall. They kind of look like they’re fixed against the same bit of the zodiac. However, if you look at them night after night you’ll actually notice that they’re moving slowly across the ecliptic–some faster than others. The moon, for example, is pretty darn zippy taking a little under a month to make one circuit. The sun takes a year to do a circuit of the zodiac. Makes sense, right? All of this is just a matter of perspective though. The sun isn’t really moving across the stars as much as the point of view from Earth is changing. This is happening such that it looks like the sun is moving across the ecliptic.
OK, so Venus is closer to the sun that Earth, right? Because of this, from Earth it looks like Venus is never more than around 45° away from where the sun is. Most of the time, Venus just moves forward along the ecliptic. If it’s located before the sun where it rises and sets before the sun rises and sets then it’s visible in the morning. From our perspective, Venus moves forward bit by bit until it overtakes the sun and starts coming after it, where it rises and sets after the sun rises and sets. That’s when it’s visible in the evening. These days, it’s visible in the evening.
Every 18 months or so, due to the Earth overtaking Venus in its orbit and the geometry of it all, Venus will appear to start backtracking. When it does it’s actually pretty speedy: it seems to take about 1.5 months for it to go from appearing far after the sun to appearing far before the sun. After that, it goes back into direct motion and starts plodding along to overtake the sun once again. We’re due for a Venus retrograde period in a few days.
So, we only notice Venus as a morning or evening star if only if it’s far enough away from the sun. Otherwise, the brightness of the sun kind of just drowns out the glimmer of the planet. I guess that’s why I haven’t noticed it as an evening star in lately up until recently. By mid-April it will have raced to appear before the sun such that it will be seen as a morning star.
Anyway, that’s my explanation. The movement of the planets is really quite a beautiful thing. Maybe over the next few evenings you’ll be able to look up and appreciate the evening star a little bit more.
Recent Comments