Binocular Brightness
Why does the apparent magnitude of Mercury change so quickly?
I am fascinated by astronomy, and two days ago I saw Mercury for the first time with unaided eye, it wasn’t much to look at, just like a dim star in the evening twilight close to much brighter Venus.
I thought to myself (excuse the informality): It’s going to stay pretty much in the same position and the same brightness in the next few days so if I can get hold of some binoculars hopefully I can take a closer look.
But using Stellarium planetary software I noticed the apparent magnitude of the planet changes a lot, when I looked it was -0.91, yesterday it was -0.84, today it is -0.76 and tomorrow it is -0.67.
I always thought the position of Earth relative to Mercury and the position of Mercury relative to the sun in our sky could not change so much in just a couple of days to affect its brightness that much.
So what makes it change so much?
Thanks.
Well, …, a few factors govern it’s apparent magnitude
1) Proximity to the sun. The Sun wipes a lot of Mercury’s brightness.
2) Relative position. As Mercury travels out from the Sun and closer to earth it will appear brighter and brighter.
3) Phase: It’s phase is moving from crescent to gibbous. Basically, since more of the planet’s surface appears illuminated (at least from our perspective), it appears brighter. BTW, since both Mercury and Venus are “inferior” planes (inside of Earth’s orbit) we can never see them as Full.
Finally, it changes so quickly because it’s orbital period is so fast. Only 88 days! Believe me it will diminish as quickly as it brightened.
Good luck with your observing. You’ve asked some very good questions. Keep it up!
Canon 10×42 L IS WP Binoculars 10×20 Review Microglobe.co.uk
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