All Questions
51
questions
0
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1
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69
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Clarification regarding the meaning of Universal Time UT1
I've been reading the book "From Sundials to Atomic Clocks: Understanding Time and Frequency" by James Jespersen and Jane Fitz-Randolph which is available at https://www.nist.gov/system/...
10
votes
4
answers
3k
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If we had just invented the first clock, and we only had a calendar system, how would we set the time of day for the first time?
I've noticed there are extensive answers on this website about the accuracy of atomic clocks and how they reference the time between each other with the average of time between each other, but I ...
- 409
2
votes
2
answers
68
views
What definition of now is used in our stellar neighbourhood?
Within General Relativity the idea of simultaneity is fairly arbitrary, every coordinate system has one. Which one corresponds to my personal local sense of now appears indefinable in any objective ...
- 2,155
1
vote
2
answers
116
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"Now" in galaxies billions of light years away
Lets say we see a red giant that has 10 million years left before going supernova, and it is 100 million light years away from Earth. Would it be incorrect to say that now, that star has long since ...
- 155
12
votes
6
answers
6k
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How could Tycho Brahe determine positions without accurate clocks?
Tycho Brahe determined the positions of stars and planets to an accuracy of 2 minutes of angle. Pendulum clocks hadn't been invented yet so he couldn't have known the time to better than 15 minutes. ...
- 165
1
vote
1
answer
145
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Confusion with light year distance and space events
I had read that the Betelguese star may be going supernova in may of 2022. Now it is about 500 Light years away, so if we see a supernova explosion in may, then does that mean it occurred about 500 ...
- 2,042
22
votes
5
answers
3k
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When we say 'Andromeda galaxy is 2 500 000 light years away from us' do we mean 'now' or in a far past?
When we say 'Andromeda galaxy is 2 500 000 light years away from us do we mean 'now' or in a far past and can this past be calculated easily?
- 2,255
16
votes
8
answers
3k
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Why is there one fewer solar day per year than there are sidereal days?
solar day = time between solar noons
sidereal day = period of Earth's spin
Wikipedia says "relative to the stars, the Sun appears to move around Earth once per year. Therefore, there is one fewer ...
- 411
17
votes
1
answer
2k
views
Is Earth’s rotation slowing down or speeding up?
Section ‘Slowing down of the Earth’ of the Wikipedia article Leap second has the following paragraphs:
A mathematical model of the variations in the length of the solar day was developed by F. R. ...
- 271
0
votes
1
answer
168
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How to calculate Night Flight time during a flight? [closed]
I am currently working on a web app (Pilot Logbook) that requires a user to automatically calculate the period of flight spent in the night.
Currently, I have
Departure Lat and Long and Departure ...
- 103
1
vote
1
answer
125
views
Does the Hubble constant measured for huge distances mean it is not the actual Hubble constant but a very old one?
I am citing Wikipedia about Hubble constant....
"Hubble constant is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec (3.09×1019 km) away, and its value ...
- 2,255
0
votes
1
answer
251
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When it's noon on the Moon, then what time is it here?
Is there a way to calculate the time it has to be on Earth at any moment in time when a person on the Moon would say their clock reads noon (i.e. a time at which a sundial on the Moon would cast no ...
- 309
21
votes
7
answers
5k
views
Finding how much time it takes for a complete Earth revolution around the Sun
Long story short, my brother made a joke about how stupid it is to celebrate the Earth making one "trip" around the Sun: New Year's Eve.
So I got curious and was wondering: how could the first ...
- 320
0
votes
0
answers
41
views
Solar panel and Equinox
At Equinox the number of hours of daylight and night is equal so 12 hours of each...
is this from 6am to 6pm? how can we know which hours this daylight happens for.
The question is:
Over which ...
20
votes
4
answers
14k
views
How can a day be exactly 24 hours long? [closed]
The longest solar day of year is approximately 24 hours 0 min 30 seconds (occurs at mid to late December) while the shortest solar day of year is approximately 23 hour 59min 38 seconds. If I average ...
- 1,005