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, ...

asked by bgusach 24 votes
answered by bgusach 22 votes

Was this an astronomical phenomenon observed in 1689?

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 ...

asked by pst 21 votes
answered by ELNJ 24 votes

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 ...

asked by Magnus W 9 votes
answered by Tim Campbell 17 votes

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

The calculator on the website 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  
asked by Helena Wells 6 votes
answered by James K 16 votes

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?

asked by Joe Jobs 6 votes
answered by James K 8 votes

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  
asked by Milano 6 votes
answered by James K 17 votes

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  
asked by Prith 5 votes

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  
asked by wim 10 votes
answered by Stan Liou 20 votes

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 ...

asked by ScottF 5 votes
answered by Florin Andrei 6 votes

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  
asked by John Canon 18 votes
answered by Tosic 54 votes

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 ...

solar-system space comets radiation dust  
asked by anonemes11 36 votes
answered by James K 53 votes

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  
asked by hawaii 17 votes
answered by David Hammen 28 votes

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  
asked by S. Kohn 50 votes
answered by astrosnapper 69 votes

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  
asked by Asadullah Ali 20 votes
answered by TildalWave 15 votes

Can you answer these questions?

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  
asked by wrb98 2 votes
answered by ELNJ 0 votes

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 ...

amateur-observing adaptive-optics  
asked by pmac 4 votes

How significantly does the intensity of a meteor shower diminish after the peak?

Every August 12 I take the family to the desert to enjoy the Perseid meteor shower. This year there will be an especially bright moon rising before 1:00, which is when the shower really peaks. Last ...

asked by dotancohen 3 votes
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