Astronomy Stack Exchange Community Digest

Top new questions this week:

Is it possible to completely see the edge of a half moon at night?

Some days ago, about 1 ~ 1.5 hours after the sunset I could see a very bright half moon, and the interesting part is, I think I could see the edge of the dark half of the moon, i.e. a whole circle, ...

the-moon

In a Swedish church record from 1689 a phenomenon in the sky is described. With my very limited understanding it could be a meteorological or an astronomical phenomenon, so here I ask if it could have ...

amateur-observing

When the sun "enters" a constellation, doesn't it make it impossible to see that constellation?

I was reading on NASA's website about how the Babylonians came up with dividing the sky into different slices and picked a pattern of stars in each slice to represent it. This ties in to astrological ...

constellations

Have I nearly found the event horizon of a black hole?

The calculator on the website https://planetcalc.com/1758/ cites Wikipedia's Gravitational acceleration and implements: $$g = G \frac{M}{(R + h)^2}$$ For a black hole with mass $M$ equal to 5 solar ...

black-hole gravity astrophysics general-relativity

Is there permanent dry ice on Mars?

Are there any places featuring permanent dry ice (carbon dioxide ice) on Mars? Is there dry ice at the poles?

mars

Stargazing (Tenerife) - different quality today and yesterday

I'm on the Tenerife island and (in the mountains above the clouds - 2300) the day before yesterday, I could clearly see the Milky Way and stars (1 am). So I went to the same place yesterday and the ...

amateur-observing star-gazing

What are the Mathematical Algorithms/Formulas behind hindu Sunrise and Sunset Calculations?

There are differences between Astronomical Sunrise/Sunset calculation and Hindu calendar Sunrise/Sunset calculations. The astronomical algorithms to calculate the sunrise/sunset can be found over the ...

amateur-observing positional-astronomy

Greatest hits from previous weeks:

Years, months, days, and ... weeks?

Why do we divide time into weeks? Is there any celestial reason why humans do this? one year: earth revolution around the sun one month: moon revolution around the earth one week: 7 days = ??? ...

planet time

Is the sun any brighter during a solar eclipse

I keep hearing that you can go blind during a solar eclipse. Is the sun actually any brighter during a solar eclipse? This is how the media seems to frame it, and they suggest you need special ...

solar-eclipse

Moon's unusual gravity

We know that the Moon's gravity is about one-sixth that of Earth. Then I recently read that the mass of the Moon is about one-eightieth the mass of Earth. Since gravity depends on the mass of the 2 ...

the-moon gravity mass

How can comets have tails if there's no air resistance in space?

I understand that solar radiation causes material to vaporize out of a comet into dust but why does the dust then trail behind the comet like a "tail"? Assuming gravity is the only applied force ...

Will Earth lose the Moon before the Sun goes into supernova?

Ive read on some sites and saw on youtube videos that the moon is getting away from earth by 1-3 cm a year. Is this enough to make the Earth lose the Moon before the Sun goes into Supernova? Im ...

the-moon the-sun gravity earth supernova

Why does it take so long to transmit an image from New Horizons to Earth?

I just got the news that the New Horizons space probe has passed by some remote planet on the edge of the solar system. I was surprised that the guy from NASA says that it might take 24 months from ...

solar-system data-analysis nasa

What is the distance that the Moon travels during one orbit around the Earth?

Also, does it always take the same amount of time, or does it fractionally differ on each revolution?

the-moon orbit earth distances

Strip Brightness in a Spherical System

My question concerns the following problem, published in Galactic Dynamics by Binney and Tremaine and based on a work by Schwarzschild. Specifically, I am having difficulties in deriving the given ...

galaxy luminosity galactic-dynamics galaxy-cluster brightness

Deformable optics for consumer telescopes

I am aware that there are adaptive imaging techniques for amateur astronomers. But these methods correct an image on a screen. This is nice, but to me the real lure of amateur astronomy is the act of ...