APOD: Mercury and the Quiet Sun (2019 Nov 14)

[Chris Peterson]

Thanks for the reply, Chris. Several items of helpful info.

One in particular would seem to rule out the "interstellar medium" idea I put forth.

... I doubt that the interstellar medium, or anything lying between stars and us, represents a measurable component of variability. There's no correlation between the light curves of very close stars, as you'd expect for something in the interstellar medium.

I think you're saying that there's no correlation between the variability for star light curves and their distance from us (in our galaxy, on the scales we're considering). I that correct? If that has been looked at and it has been found that there is no such correlation, that's a key fact.

If variation was caused by intervening material, you'd expect to find correlation in the light curves of stars that are located close together in the sky, regardless of their actual distances (although stars that were at the same distance would correlate better... such as double stars). I know that the light curves of double stars don't show correlation.

[MarkBour]

...
If variation was caused by intervening material, you'd expect to find correlation in the light curves of stars that are located close together in the sky, regardless of their actual distances (although stars that were at the same distance would correlate better... such as double stars). I know that the light curves of double stars don't show correlation.

Ah, even better. I had not thought about how double stars could inform such a question. Most of the thoughts I had about what might be external to the star, but a cause of variability, would seem to be ruled out by this fact.

[BillLee]

Can somebody explain why there will only be 14 transits in the 21st century. Seems to me that every time Mercury goes around the sun that it would be visible against the sun as a backdrop. I cannot envision the geometry that would limit that to only 14 in 100 years.

Mercury's orbit doesn't lie on the same plane as Earth's orbit. Most of the times that Mercury lies between the Earth and the Sun it is tilted enough above or below Earth's orbital plane that it isn't passing in front of the Sun's disc. It's the same reason we don't have a solar eclipse and a lunar eclipse every month (because the Moon's orbit around Earth isn't coplanar with Earth's around the Sun).

Thanks, all, for the explanation. I knew about the odd plane for Mercury, just didn't know it was enough to throw it off the Sun's disk when between Earth and the Sun.