Questions tagged [time]

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

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7
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1answer
118 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
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1answer
95 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
377 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
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3answers
620 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
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1answer
731 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
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1answer
154 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
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1answer
4k 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
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1answer
394 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
61 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 ...
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1answer
83 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 ?
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1answer
306 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
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2answers
267 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
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3answers
205 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 ...
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2answers
101 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 ...
5
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1answer
907 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 ...
3
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2answers
304 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 (+-...
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1answer
678 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 ...
4
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2answers
139 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
73 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 ...
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2answers
5k 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 ...
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3answers
249 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 ...
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0answers
3k 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? ...
3
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1answer
554 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 ...
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2answers
598 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 ...
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4answers
535 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 ...
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4answers
721 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 ...
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1answer
182 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....
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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 ...
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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
475 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 ...
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2answers
178 views

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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1answer
99 views

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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1answer
703 views

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 ...
5
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1answer
238 views

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 ...
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0answers
63 views

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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3answers
5k views

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 ...
6
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2answers
127 views

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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4answers
4k views

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 ...
4
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1answer
204 views

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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1answer
84 views

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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1answer
670 views

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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1answer
74 views

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 ...
5
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1answer
595 views

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 ...
3
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1answer
188 views

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 ...
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3answers
12k views

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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2answers
2k views

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 ...
2
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1answer
544 views

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 ...
0
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1answer
1k views

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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1answer
226 views

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 ...
2
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2answers
90 views

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 ...