Questions tagged [time]
Questions on time, the indefinite progress of events in the Universe.
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33 views
Syncing time for measuring the parallax of Mars
I read somewhere (I can't remember the website) that when Cassini and Jean Richer measured the parallax of Mars, they used Jupiter's (or maybe Saturn's) moons to sync their time. How was this done?
4
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0answers
56 views
Why does “longitude by chronometer” require first calculating latitude?
I'm studying the history of longitude and Greenwich Time, and I'm currently confused about the "longitude by chronometer" technique.
Conceptually, the method seems extremely simple. With a ...
0
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3answers
151 views
How can we see stars if they are hundred years of light far from the Earth?
So the right question is, is the light years counted from that second when you look up to the sky? If that so, how can we see the stars from the telescope (which I think it starts when you put your ...
2
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1answer
39 views
How do I calculate Allan Variance (maybe for pulsars)?
I am studying pulsar timing this summer and have come across many Allan Deviation plots that include several popular clocks alongside pulsar measurements, like this one by N. Ashby and D.A. Howe:
I ...
1
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0answers
35 views
What exactly is a Right Ascension clock and how was one (mounted on a telescope) used in practice, historically?
@MikeG's answer to Why does this Lowell Observatory telescope have so many knobs? What do they all do? explains that item #6 labeled in the image there (and cropped version here) is likely to be a "...
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2answers
69 views
What are some examples of routine human activity times for certain cultures/peopes that are based on the position of the sun?
What are some examples of timekeeping that are based on the height/angle/position of the sun. Two examples that come to mind are the times of the Islamic prayers times and the golden hour in ...
2
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0answers
53 views
Is TDB monotonic? Does it or any of its derivatives have yearly discontinuities with respect to an atomic clock?
This in-depth answer mentions that
...TDB is updated every year. As technology improves, these subtle updates change the timing of past events.
This makes me wonder...
Does this updating result ...
2
votes
1answer
63 views
Validity of ephemeris time
In the wikipedia article for epmeris time as first adopted in 1952, it does not explain clearly how or why it was supersed
ed. For example, it reads:
In 1976 the IAU resolved that the theoretical ...
7
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1answer
166 views
Universe is expanding, why doesn't time expand with it?
Reading this response When the universe expands does it create new space, matter, or something else? I understand Universe is not expanding like if it is stretching its space, but it is actually ...
3
votes
1answer
94 views
Sorting out Julian Day, Julian Date, Julian Day number, Julian Day Calendar, and Julian Day Table
In this answer I mention day number which is 1 on the first day of each calendar year (January 1) and increments to 365 or 366 on December 31 of that year.
There was an edit proposed, which included ...
1
vote
1answer
78 views
Is there a maximum Julian day?
I have read various discussions of the Julian day, including that on the Wikipedia, but none of these appear to define the maximum Julian day or proleptic Gregorian year in which that maximum day ...
2
votes
2answers
112 views
Difference between zone time and local sidereal time?
What is the key difference between LST and ZT?
Also, are UT, GMT and Greenwich Mean Solar Time the same?
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3answers
125 views
What orbit is the smallest or takes the least time to complete?
What orbit in our solar system would take the least time to perform 1 orbit?
What is the fastest possible orbit to complete in the universe?
0
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3answers
107 views
If FTL travel is possible can i look back in time to a younger earth? [closed]
For this question assume that.
We found a way to go faster than light.
I build a ship that could go faster than light and go to another planet with this ship in the span of mere hours. The ship was ...
2
votes
1answer
91 views
Why does gravity make time go slower and warp light?
I know that objects with lots of gravity are capable of warping light or slowing time (an example being a black hole). I also know this is because of the theory of relativity, but I need someone to ...
2
votes
1answer
109 views
How far does the Earth travel around the Sun during each month of the year?
After reading about the Earth's orbit and how it moves faster in January than it does in July (based on its distance from the Sun), I started to wonder...
How far does the Earth travel around the Sun ...
3
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0answers
40 views
How did the Big Bang occured if before that there was no time? [duplicate]
I am thinking, that if the Big Bang created time, it implies that before the Big Bang there was no time (correct me if I am wrong).
My question is that, if there was no time before the Big Bang, how ...
2
votes
1answer
115 views
Determine Julian Date from Gregorian without formula
As an exercise, I am trying to calculate the JD from a given date in the Gregorian Calendar at a time given in UT. Furthermore, I want to do so from first principles without relying on any formula. ...
21
votes
1answer
3k views
What's the reason that we have a different number of days each month?
It always was interesting for me to understand the answer for the question:
What's the reason that we have a different number of days each month?
If the month is fixed on the time that the moon ...
2
votes
1answer
60 views
What causes random irregularities in the earth's rotation accounted for by dynamical time
I read in a book that the earth's rotation sometimes shows unpredictable irregularities(not caused by precession, nutation, tidal forces) which are accounted for using dynamical time. What would be ...
3
votes
1answer
386 views
How to convert sidereal time to local time?
I came across something that won't let me go, but no matter how much I calculate (which I do not like), it doesn't seem to work.
Also: I've used so much apps and links already to convert, but it doesn'...
3
votes
3answers
175 views
Is this a coincidence?
The smallest distance probed in any experiment so far conducted is $10^{-18}$m and the largest distance we can have is the radius of the universe, nearly $10^{26}$m. The ratio is $10^{44}$.
In ...
0
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2answers
129 views
Does the Milky Way's movement through space have spacetime implications?
The way I usually see spacetime explained, it says that if you just sit still, you are moving at 0kph and so you are moving at maximum speed through time (ie. time for you is flowing at maximum speed)....
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0answers
47 views
Can you go blind if you look at the sunrise and sunset? [duplicate]
Everyine knows its dangerous to look directly at the sun, but can you look safely at sunrises and sunsets without going blind? Because the uv radiaton and the light is much weaker. Or is it still very ...
4
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4answers
255 views
What is this “time travel” these popular blogs refer to?
I am getting confused by some "popular science" about time travel, such as
or "Time Travel Is Real. Here Are the People and Spacecraft Who Have Done It":
It is indeed an interesting property of ...
7
votes
1answer
172 views
Why aren't GPS clocks sufficient to synchronize VLBI observations?
In very long baseline interferometry, it is typical to use a hydrogen maser atomic clock to sample data. However, given how expensive these atomic clocks are, it seems surprising that more ...
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2answers
168 views
calculating rising time from culmination time
I want to know how to calculate the rising time of a star, if
culmination and setting times are available with me?
Say Culmination time is 03:00 on 28-Nov-2018
and Setting time is 05:30 on 28-Nov-...
3
votes
1answer
234 views
In the calculation of GMST, what are these constants? [duplicate]
GMST = 6.697374558 + 0.06570982441908 D0 + 1.00273790935 H + 0.000026 T2
From the GMST calculation on the USNO website (and many other places) we encounter these constants:
6.697374558
0....
3
votes
2answers
170 views
How is distance in light-years converted to years back in time?
When I look at the Alnitak,the left most star of Orion's belt, it is 736 light years away. How could I convert this distance to an estimate of how long ago what I am seeing happened.
Would it be ...
3
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0answers
43 views
How does the geometry for constructing a declining vertical sundial work?
I understand how to construct South-facing vertical an horizontal sundials and the geometry behind their construction by projecting an equatorial sundial into a plane, but I can't quite get how to do ...
0
votes
2answers
79 views
Existence of a “reflecting” star [closed]
Wasn't even sure about what the title should be, but I had a thought and I wonder whether it may be possible or not.
Is it possible that there is a star which acts as a mirror and reflects back the ...
1
vote
1answer
160 views
How could time only have started with the Big Bang?
I understand that before the Big Bang time (as well as dimensions) didn't exist. But how could this be? If there was no time then nothing could change, and so time itself couldn't come into existence.
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1answer
114 views
Clarification on Greenwich Sidereal Time (GST)
I know that LST = HA + RA.
And GST is LST + longitude(L).
If a star culminates, it's HA=0. Therefore, GST = RA + L.
Since RA and L are constants, GST must also be a constant. So, how can GST be ...
1
vote
0answers
33 views
What's the time in which the sun rises? [duplicate]
What's the time the sun takes in order to rise? (The time in which the solar disk crosses the horizon)
How does this time depend on local latitude and the sun's declination?
Assume that we're at sea ...
2
votes
0answers
72 views
What's a good model for the vernal equinox year? Can it increase forever?
Duncan Steel (2002) makes an interesting argument that the notion of year is (1) somewhat arbitrary and, more interestingly (2) one notion of year can increase while another decreases.
His examples ...
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0answers
107 views
Is the Milankovitch calendar provably better than the Gregorian one considering a non-constant tropical year?
It's somewhat of a no-brainer the revised Julian calendar (attibuted to Milankovitch) is more accurate than the Gregorian one just because it started with a more accurate estimate of the duration of ...
4
votes
1answer
584 views
Does Time Exist for Photons? [closed]
If time slows down the closer you get to the speed of light, does it follow that time does not exist for photons. Do photons - even if they travel across billions of light years of space - arrive at ...
1
vote
1answer
50 views
Rising & Setting time of a star on a specific date
Given the local sidereal time (LST) of a star when it rises, how can I find out the time when it will rise (according to my watch) on a specific date?
7
votes
1answer
113 views
What are the problems that need to be taken in consideration when trying to find planetary position by using jpl ephemerides?
I'm currently trying to determine the position of the sun and the moon from jpl ephemeris DE200 which is referred to the dynamical equator and equinox of 2000 and uses tdb time. I am using the j2000 ...
3
votes
1answer
87 views
How are mars years counted?
In this diagram about the methane concentration in the martian atmosphere, there are data points labeled "Mars year 32", "Mars year 33", and "Mars year 34". How are Mars years counted? Is there a "...
2
votes
2answers
277 views
Computation of mean sidereal time at Greenwich using solar position algorithm
Using Equation 28 in Section 3.8.1 of Solar Position Algorithm (Reda, I. and A. Andreas, Jan. 2008), I computed the mean sidereal time at Greenwich for 2003 October 17 for four different times (UT), ...
7
votes
3answers
563 views
Does Universal Time really track mean solar time?
At innumerable places on the net you can find the claim that leap seconds are inserted into UTC in order to make it track UT1 which again tracks mean solar time at the reference meridian.
However, ...
6
votes
1answer
578 views
Effect of the obliquity of the ecliptic / tilt of the Earth on the equation of time
In an attempting to answer the question “why aren’t changes in sunset and sunrise times symmetrical around the solstices ?” I’ve got stuck on being able to explain the effect of the obliquity of the ...
4
votes
1answer
143 views
Do today's professional astronomers use earth rotation angle or sidereal time?
I have read that "earth rotation angle" is the modern replacement for "Greenwich sidereal time". Is it actually the case that modern professional astronomers are using earth rotation angle? Either way,...
8
votes
1answer
3k views
Local Sidereal Time
I am trying to understand how to calculate local sidereal time and have found the following formula:
$$\text{LST} = 100.46 + 0.985647 \cdot d + \text{long} + 15 \cdot \text{UT}$$
Here, $d$ is the ...
3
votes
1answer
335 views
How do I adjust the sunrise equation to account for elevation?
I have been working with the sunrise and sunset formula, using the equations from Wikipedia. The problem is that, in my usage thereof, it seems that it only works for sea level; if I start with ...
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1answer
52 views
How can I calculate how far through the day the prime meridian is of different planets
I'm trying to create a widget that shows the 'time' on different planets. It will show how far through the day/night cycle (as a percentage) a point on the planet is. It has been easy to scale down ...
0
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1answer
80 views
Time travel in past and future [closed]
Why it is not even theoretically possible to travel in past but possible to travel in future ?
-1
votes
1answer
277 views
At which winter hour is sun in zenith in DST?
When DST is introduced does it mean that the sun is in zenith one hour later (1pm winter local time) or one hour earlier (11am winter local time)?
Edit: The location is New York.
5
votes
2answers
254 views
Why isn't the day backwards between leap years?
So every four years, we add an extra day. A leap year. This is because a year is actually $365_{1/4}$ days long. But what happens between leap years? After two years, it would get dark at 8:00 AM ...
