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Questions tagged [time]

Questions on time, the indefinite progress of events in the Universe.

2
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1answer
32 views

What's the reason that we have different quantities of days each month?

It always was interesting for me to understand the answer for the question: What's the reason that we have different quantities of days each month? If the month is fixed on the time that the moon ...
2
votes
1answer
51 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 ...
2
votes
1answer
59 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
157 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
votes
2answers
101 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)....
1
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0answers
44 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
243 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
112 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 ...
1
vote
2answers
49 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
80 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
152 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
36 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 ...
-1
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2answers
73 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
votes
1answer
102 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.
1
vote
1answer
44 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
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0answers
32 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
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0answers
44 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 ...
1
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0answers
72 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
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1answer
554 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
38 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
101 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
68 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
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2answers
201 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
427 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
414 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
121 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,...
7
votes
1answer
2k 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
238 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 ...
1
vote
1answer
46 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
votes
1answer
67 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
168 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.
6
votes
2answers
234 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 ...
3
votes
3answers
168 views

Knowing time any where and any when

My question is if I put you down somewhere in the Milky Way galaxy in any given period of time, is there a way for you to tell when and where you are based on stars or constellations? Or some known ...
1
vote
2answers
93 views

Can we, with arbitrary precision, calculate what's the place on earth that's closest to the sun on a given day?

Can we, with arbitrary precision, calculate what's the place on earth that's closest to the sun on a given date? If so, what's the math like? Say, if I wanted to calculate this within 1m, 10cm or 1cm ...
4
votes
1answer
432 views

What is the difference between Terrestrial Time (TT) and International Atomic Time (TAI)?

The equation which relates Terrestrial [Dynamical] Time (TT, or TDT) and International Atomic Time (TAI) is the following, according to the U.S. Naval Observatory: $TDT = TAI + 32.184$ As I have ...
2
votes
1answer
163 views

Getting Solar Noon from Sunset and Sunrise

I've seen the solar noon formula described as snoon = 720 – 4*longitude – eqtime. I'm curious if it is equally reliable within a relatively small margin of error (+-...
1
vote
1answer
487 views

Sidereal time calculation

I am creating program in C which is derived from Javascript Solar Radiation calculator based on a book Guide to HTML, JavaScript and PHP: For Scientists and Engineers By David R. Brooks. The original ...
1
vote
1answer
79 views

Is there a timekeeping word for the orbit of a moon?

For a planet, we can colloquially refer to its period of rotation as a "day" and its period of revolution around its parent star as a "year." Some worlds have unique terms, such as Martian days being ...
2
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1answer
67 views

Which worlds have unique words for a “day”?

On Mars, we refer to a "day" as a "sol". NASA has developed a whole vocabulary around this term, like "yestersol" and "tosol", etc. Do any other worlds have unique terms like this? How would one ...
1
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2answers
2k views

How to find Greenwich Mean Sideral Time?

I'm writing a program that converts position in the ECI (CIS, EPOCH JD2000.0) coordinate system to WGS 84 (CTS, ECEF) coordinates. To do so, I'm following what's described in this document and ...
2
votes
3answers
198 views

Why the continued obsession in measuring the year as a multiple of days?

Pick a any notion of year and any type of day measurement. Outside of history and the common cycle of day an night, why are these viewed as integrally or even fractionally commensurate? Why all the ...
1
vote
0answers
2k views

How do you calculate local time from local sidereal time?

There are many calculators online to convert local time to local sidereal time, but I can't find any resources to convert the other way. Can you convert back by doing the same conversion backwards? ...
2
votes
1answer
255 views

“Official” time for other planets

I want to make clock that shows current time on different solar bodies. For that I need to know how to count time on those planets. Is there any official definition of times on other planets in our ...
0
votes
2answers
460 views

Difference of sunset and sunrise of two cities are different

For 26 Nov 2016: Consider Tehran with 06:51 sunrise and 16:52 sunset. Another city is Rafsanjan in 850KM southeast of Tehran, sunrise is 6:21 and 16:45 for sunset. Time zone of two cities is +3.5 ...
0
votes
4answers
322 views

Obtaining deltaT for use in software

I'm currently developing a javascript application in which I want to calculate the approximate position of the sun. This works quite fine but requires the value for deltaT (TT-UT) to be set depending ...
1
vote
3answers
355 views

What time and where on earth is the latest solar noon?

For context: I am visiting Portugal from Australia and after observing that people tend to start their days later, I realised that solar noon also seems to occur at a later time - about 1.20pm at the ...
0
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1answer
133 views

Local sidereal time interpretation

I am new to this domain and trying to understanding the fundamentals of local sidereal time. Through an online available calculator https://www.iiap.res.in/personnel/reks/software/javascript/calclst....
4
votes
2answers
2k views

Why is JD 2451545.0 = January 1, 2000, Noon instead of JD 2451558.0?

I know that JD0.0 is Jan. 1st 4713 BC at Noon, a year in the Julian calendar is 365.25 days and that the number after 'JD' is the number of days so... I want to calculate the number of days between ...
16
votes
3answers
7k views

Does the moon have days?

As like Earth, does the moon have a day (24 hours). If it has, how many hours are in a moon day?.
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0answers
399 views

How is the time of sunrise and sunset determined?

If I do a quick Google search for amsterdam sunrise, I immediately get the time back: 6:42 AM. How is this time calculated? (I ...