Questions tagged [time]

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

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44 views

Encode place and date in star pattern

Disclaimer: I'm an astronomy noob You sometimes see in (B?) movies, that they infer dates from sky patterns. I was asking myself if you can encode a place and date in a mostly meaningful pattern? ...
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140 views

How is the Gould Belt younger than the sun?

Our sun is located in the Gould Belt, a group of stars which is thought to be 30 - 50 million years old. However, the sun formed 4.6 billion years ago. Did the belt form around us?
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What is the exact time of sun stay in each zodiac every year on sidereal year duration?

I tried to reverse engineer from astro-chart times at which sun stays in a zodiac. Here is the list. ...
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2answers
108 views

Does mean solar time and sidereal time sometimes indicate the same time?

I'm trying to wrap my head around the different definitions of time. Since mean solar time depends on the Sun, and sidereal time depends on the stars, and since the position of the Sun relative to the ...
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73 views

How can I calculate 27 tropical/solar year since my birthdate?

Let's say I was born on 27th November 1993 at 03:15:15 AM. Is there any way I can calculate exact 27 tropical cycle since my birthdate/birthtime. What date and what time it will be after exact 27 ...
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72 views

Are there predictions of the distribution of matter at the end of time

Are there any theoretical predictions or "educated speculation" by cosmologists on how matter will be distributed at very, very distant times in the future? For example, will matter be: agglutinated ...
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1answer
67 views

Is there a way to know the time according the moon appearance?

Is it possible know the time according to the moon location? To look at the sky at night and then determine what the time is.
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67 views

Can I use the Tokyo Skytree as clock?

After my observation today at the Skytree in Tokyo, I am wondering if this building could be used as a giant sundial? As an aside, it would be interesting to consider how the Skytree might be used ...
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2k views

How is time defined in astronomy?

I was amazed when my teacher told me that in space, time is define as a coordinate. It means we can define something with a four coordinate system with "negative time" $-t$ as easily as we can say $-...
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52 views

Does Gravitational Time Dilation stack on top of Time Dilation caused by Velocity?

Using 1 CM from the Event Horizon of Super Massive Black Hole, SGR A, as the reference point. The particles within the accretion disk are traveling at extreme high speeds. Assume this speed increases ...
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2k views

Coordinate system for space [closed]

I am high school student, I have doubt if I have to locate a planet or any star in space, how do I locate it, because we have no perfect coordinate system, I have read in some books, that time is ...
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35 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?
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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 ...
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3answers
165 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 ...
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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 ...
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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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71 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 ...
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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 ...
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70 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 ...
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206 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 ...
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1answer
181 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 ...
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1answer
82 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 ...
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326 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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146 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?
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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 ...
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1answer
93 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 ...
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1answer
132 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 ...
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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 ...
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1answer
157 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. ...
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1answer
4k 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 ...
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1answer
65 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 ...
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1answer
770 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'...
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185 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 ...
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145 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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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 ...
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257 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 ...
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216 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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277 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-...
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1answer
342 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....
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2answers
191 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 ...
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1answer
68 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 ...
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83 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 ...
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2answers
299 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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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 ...
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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 ...
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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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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 ...
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1answer
610 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 ...
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53 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?
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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 ...