# Questions tagged [time]

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

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### Time travel in past and future [closed]

Why it is not even theoretically possible to travel in past but possible to travel in future ?
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### 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.
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 (+-...
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1 vote
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
1 vote
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### 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 ...
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1 vote
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### 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? ...
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### "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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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....
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### 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 ...
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### 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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### 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 ...
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### Deriving phase of a pulsating star from list of Julian date values of observing brightness

I want to convert the list of Julian date values of observing brightness of a Cepheid variable into its phase. The equation I found in the Internet is the following: \mathrm{Phase}= \frac{\mathrm{...
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### How accurately can you tell the time using a photo of the stars?

If I were to go out on a clear night, in an area with low light pollution, and take a photo of the stars, could an expert analyze the photo and figure out what date the photo is from? We can assume a ...
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### How Soon Could a Waxing Crescent Moon Be Seen?

Question: Apart from a Solar Eclipse, How much time is needed until a Waxing Crescent Moon be seen following a New Moon? Would the time of year be significant? The Vernal Equinox + 1 Month. Could it ...
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Imagine a photon reaching the Hubble Space Telescope today had originally been emitted from a star in the early universe 13 billion years ago. Einstein’s Special Relativity tells us, traveling at the ...
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### Is there a variation of ISO 8601 for julian calendar dates?

If I want to write a date in the Gregorian calendar, I can write it in the ISO 8601 format; today would be 2016-02-20. The advantage is that there are many parsers available to let computers read in ...
1 vote
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### About bias in sidereal time (used by astronomers)

I came up on this stanza in various websites when looking up on experimenter bias. It says "If the signal being measured is actually smaller than the rounding error and the data are over-averaged, a ...
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### How does the appearance of the analemma vary with latitude

How does the appearance of an analemma vary with latitude? This what I take to be a definition of an analemma: If the position of the sun is plotted or photographed at noon clock time (say UTC) every ...
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### September 26 twelve hours sunrise to sunset?

I noticed in today's newspaper (Boston Globe, September 26) that sunrise and sunset were both at 6:25. That's the twelve hour day I'd have expected at the equinox. (Please retag if there's a better ...
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### How can we avoid needing a leap year/second?

Given the Earth's current speed around the sun and current rate & axis of rotation, what is the best way to keep time to avoid a leap year? How many hours should we have in the day and days in a ...
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### Determining North-South Line Via Watch Method: Theory & Reason

I recently read that if you're in the northern hemisphere and have an analog watch, then you can point the hour hand at the sun and know that a south line lies between (bisection) the hour hand and ...
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1 vote
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### How could eclipse happen on the same day in a different calendar? (45 AD)

According to the eclipse calculator I use (Alcyone), a partial eclipse 0.314 magnitude occurred August 1, 45 A.D. in Rome starting at 8:35am local time. This would seem to correspond to an eclipse ...
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1 vote
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### What's the difference between the Julian and the Julian Ephemeris Day?

I'm making a code where I calculate the sun position in a given time. But I also want to learn astronomy with it. I'm following a .pdf to make the program, and in this .pdf it says The Julian Day (...
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### what is the time mentioned after date (1962-07-03+02:00) the +02:00? [closed]

i was retrieving a some personalities birthday date through computer programming when i retrieve a person's birthday the result was ...
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### How to calculate geocentric conjunction time and moon altitude at a given time

Umm al-Qura calendar of Saudi Arabia determine the new moon if these criteria meet at 29th day: The geocentric conjunction occurs before sunset. The moon sets after the sun. I've got the formula to ...
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### Day Length on Double Planets

Assume two similarly sized bodies tide-locked to one another orbiting a barycenter between the two. That barycenter orbits a star. Since the two are tide-locked, their sidereal rotational period and ...
1 vote
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### Space time and aging

Would a more significant curvature of spacetime relative to Earth (i.e. time on Jupiter) effect the aging process? Basically would you age at a slower rate? Or age faster on the Moon?
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### 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 = ??? ...
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1 vote
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### Find constellation over Earth coordinates on a specific date-time

I would like some help in finding the exact constellations, or some sort of visualization of the stars over an specific location an time. The idea is to find out the closest star or group of stars on ...
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### When will a day on Earth & Mars be the same length?

I read on Wikipedia that: As on Earth, the period of rotation of Mars (the length of its day) is slowing down. However, this effect is three orders of magnitude smaller than on Earth because the ...
37k views

### When we see the Sun, do we actually see its past?

The Sun's rays hit our eyes around 8 minutes after they are emitted from the Sun. Does this mean that the Sun that we see is always the Sun as it was some 8 minutes before? I strongly think this must ...
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### Calculate the time when Arcturus passed through the meridian

If the observer observes Arcturus passed through the meridian about 23:00:00 of 28 April. Find the time that Arcturus passed the meridian in 29 April. And what day does the Arcturus will pass the ...
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### Why do we need to add a second to 30 June and risk upsetting Internet?

We linked a physical constant (the time of one second) to a physical non constant (the rotation of the earth) so there will have to be adjustments - or not? Is it important that the UTC in one line on ...
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### How often and over what period is Earth’s rotation averaged to compute UT1?

I understand that UT1 (and for that matter UT0, UT2, etc.) are based on averages of actual earth rotation, and serve as a form of mean solar time. However it’s not clear to me when these averages are ...
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### During what time of the year can Centaurus be seen from Tokyo?

During exactly what dates of the year can the Centaurus constellation be seen from Tokyo, Japan? And what is the ideal date for viewing it from Tokyo? I found on a website that it can been seen May-...
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### Is it possible for a person to not see the new moon at different places on earth?

It is known that theoretically we see the same moon phase everywhere on the earth, but practically I see that people are unable to observe the 0.8% waxing crescent at different places on earth. This ...
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