# Questions tagged [time]

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

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### How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?

Google says the year is exactly 365.2422 days, and so they make a leap year every 4th year, but that ends up being 365.25 days per year on average. So every 100th year they don't have a leap year, but ...
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1 vote
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### Convert GPS Week and GPS Time of Week to J2000?

I'm new to this, and am trying to convert GPS Time to J2000 time using python and Astropy. Here's what I've come up with: ...
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### How can I plot a rotary cam for the equation of time? [closed]

I'd like to use Solidworks to create a cam (peanut looking) for computing the equation of time in a pocket watch. Would appreciate any point to the right direction or advice. Audemars Piguet EoT
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### Delta T used by US Naval Observatory

I'm trying to compare some eclipse calculations to that by the U.S. Naval Observatory, specifically, these results. One thing I just don't understand is how they come up with a Delta T of 72.8s. ...
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### Determining Vernal Equinox by observation

Medieval society determined the date of Easter (or Passover) relative to the Full (or New) Moon on or after the (day of the) Vernal Equinox, where the state of the Moon was observed rather than ...
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### What year will Gamma Cephei become the North Star?

I tried googling but I couldn't get any exact year, just any time between 3000 and 3100 CE. I want to know the exact year the north pole becomes closer to Gamma Cephei than Polaris. Apparently Polaris ...
1 vote
130 views

### Determing timestamp of a photo of the stars using a plate solver

This question from 2016 seems to suggest that you need a powerful telescope to get a photo of the stars in such a way that you can use their proper motion to get a timestamp. If this is possible, how ...
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### Convert GST to UT while avoiding ambiguity

Duffett-Smith, in 'Practical Astronomy With Your Calculator', says of converting a given Greenwich mean sidereal time into the corresponding universal time. The problem is complicated, however, by ...
1 vote
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### Determining Time Based on Arc Seconds

So our arc day starts at 20°41'32 which is midnight (0:00) and ends 24 hours later at 21°42'42. According to my calculations it traveled 0 degrees, 59 minutes, and 30 seconds or 3570 arc seconds in 24 ...
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### Understanding the Julian Date equation in detail

I am trying to figure out the Julian date equation by myself. Unfortunately, there are not many resources I could find which explains in depth. Moreover, there are several variations of the same ...
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### How to find the constellation in which the sun is located using the night sky map?

The following sky map was given as part of practice problems in my Astronomy course The moon is in its first quarter phase. I am supposed to find the approximate date and time of the year in which ...
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### Asteroid in Deep Space 🌌

Suppose a asteroid in deep space and there is nothing around it to compare it with. My question is how would you known that the asteroid is moving if there is nothing to compare it with
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### How to even approach this type of question

Can someone tell me how one would approach this type of question. is there an equation we would use or something of the sort. Ths solution to this question is given below but I just want to understand ...
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### Is it possible (and if so where can I find one) to buy a sidereal wall clock or kitchen clock?

I've done some research and can't find one of these anywhere but would be dumbfounded if they didn't exist, surely sidereal wall clocks are sold somewhere (preferably online)?
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### Why does the tropical year have a ~161 years cycle?

Using PyEphem, I calculated the time difference between consecutive summer solstices in the south hemisphere for the last 4000 years, and plotted them in a graph. It seems like random noise around the ...
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### Following Meeus's Astronomical Algorithms for the Calculation of JD

For the Calculation of Julian Day (JD), I followed Chapter 7 of Meeus's Astronomical Algorithms. I could get the numbers mentioned in Example 7.a and 7.b. On the basis of these methodology, proceeded ...
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### Converting UTC (10 digit) to LST (Local Sidereal Time)

I am an astro-particle physicist looking for some simulation based on the experiment observation. The simulation dataset contains 10 digit Epoch dataset for UTC time, however, I need to convert this ...
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### Why are there not a whole number of solar days in a solar year?

I've been trying to find an answer to this question for some time now, and I seem to be missing something. I understand that the rotation and revolution of the Earth are not synchronized. I understand ...
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### Earth's Rotation & Longitude

I live in Townsville Australia which has a longitude of 146 degrees East. I'm trying to get my head around how long the Sun takes to move from Greenwich to my latitude. I have a world globe in front ...
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1 vote
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### How do I find out when sidereal time was 13:30 on a certain date and location?

I live in Lehi, Utah, USA and I want to know what time of day it was on sidereal time on February 25th, 2023. Someone said it was 3:32 AM and I wanted to check and see if that was true.
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### How is Earth's Rotation Angle (ERA) defined and measured

UTC typically ticks with TAI. But to keep UTC from drifting far from the rotation angle of the Earth we occasionally add leap seconds to ensure that UTC does not drift too far from the UT1 timescale. ...
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### What are the twinkling stars in the New Horizons Arrokoth approach?

I've circled them in this edit: Source: https://commons.m.wikimedia.org/wiki/File:New_Horizons_Approach_to_Arrokoth.ogv Note, this has generated frames, see the source....
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### How exactly is UT1 measured/calculated?

Background: I am an atomic physicist so I understand atomic time pretty well. But I'm not an astrophysicist or astronomer and I know very little about astronomical measurements. I am interested in ...
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### Moonrise and Moonset time in Sydney vs Perth

I'm very confused with multiple moonrise and moonset times across Australia. For Example (According to https://timeanddate.com) Moonrise in Sydney on 22 Mar 2023 is at AEST 7:09AM which is going to ...
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### Modified Julian Date (MJD)'s epoch is 0:00 on November 17, 1858 (JD − 2400000.5) has fewer digits than JD. But what's so special about November 1858?

A week ago in the Observatory chat room @HDE226868 wrote: Happy MJD 60000, everyone! Thank you! According to Wikipedia's Julian day; variants the epoch for Reduced JD is 12:00, November 16, 1858 (...
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### What was the GPS time at J2000 epoch?

The simple version of the question is: what was the GPS time expressed in week number (pure number) and time of week (SI seconds) at the J2000 epoch? I am asking because my hand calculations differ ...
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### How to calculate Local Sidereal Time from RA of a star and Longitude of Observer

I have been banging my head against a wall trying to understand these time conversions and the relation between all of these astronomical measurements. This is the question I am currently working on, ...
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### How were sundials and moondials possible 800 years ago?

In India, there is a temple named Konark Sun Temple which is around 750 years old and made up of 100% stones and rocks, and has a chariot which is headed by 7 horses and includes the Hindu god Surya(...
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### J2000 and J2000.0 difference?

I've seen the standard epoch described as both J2000 and J2000.0. Is there any difference between the two designations?
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### Is the speed of time much slower on sun surface?

The gravity forces on Earth and Sun is different and in Interstellar, they said something like high gravity slows time. So if we say Sun is N billion years old, that must be earth time but on Sun's ...
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### Why is time not a spatial dimension? [closed]

I am a little confused about dimensions. People say we have three spatial dimensions and one temporal dimension but I don't know how this is possible. How can there be different "types" of ...
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### Light or neutrinos graze or pass through the Sun and arrive at Earth - need an expression for Sun's gravitational effect on observation direction/time

Skyfield's Github has discussion Jupiter hiccup #815 which then links back to to Non-physical gravitational deflection corrections for Solar System bodies #734. The script and plot from #815 are shown ...
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### What kind of beast is Daylight Savings Time and Standard Time?

I understand that UTC, for example, is a "time system" and that there are regions of the globe that are divided into "time zones" (offsets from UTC); but what class of thing is, ...
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### How to show that a complete revolution of Earth around the sun takes 365 1/4 days?

Seasons change regularly and day and nights also. The fact that Earth takes 365 1/4 days to complete one complete revolution was found a long way back in history, but so far I can't find any current ...
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### (Fiction) How would a time traveler locate himself with 1962 technology?

I'm a writer doing research for a time travel story; my character's origin is in 1962 and his contraption has been built into a DC-3 No fancy mechanical clock dials or precision digital readouts; when ...
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### What programs do astronomers use to ensure their observatory control computer has accurate time?

Somewhere I've seen that especially those observing asteroids, in order to provide accurate observations, use timing servers that sync to atomic clocks. What software can be used to do that?
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### Sidereal times occuring twice a day question

I'm reading Practical Astronomy With Your Calculator Or Spreadsheet by Duffett-Smith and Zwart. Discussing the conversion of GST to UT, the authors mention that because the sidereal day is slightly ...
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### Maximum mass of the observable universe

As I understand it, the maximum radius of the observable universe will be about 62.9 billion ly (at t = ∞). Does it mean that the observable universe will reach a maximum mass (baryonic + dark matter)?...
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### Is time finite or infinite? [closed]

I am not a scientist nor do I have a degree in Astrophysics, but I do like to learn new things by asking questions. With that being said, I have read that time is relative to space which began after ...
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### How many light seconds away is the JWST?

The processing for JWST's alignment is done on Earth. How long does it take for a signal from Earth to reach the JWST?
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### Calculating local solar time with python ephem library

I was learning to calculate local solar time with ephem. I must admit, unit conversion between radians, hours and days, which are all represented by a float in the ...
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### How can we see black holes collapse if time inside the event horizon is infinite?

My understanding is that if you fell into a black hole, i.e. crossing the event horizon time would be speed up infinitely fast. This means I could see my whole galaxy die, new blackholes develop and ...
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### Depth of gravitational well within our local Virgo supercluster?

For simplicity we could imagine standing at the north pole so we do not have to care about how fast we are moving in relation to the gravitational field of the earth. Now calculating how much faster ...
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### Earth’s rotation speed vs rate of lunar cycle

Hypothetical Question If the earth’s rotation slowed down to align to the lunar cycle so that the orbital period began and ended at the same lunar phase, what other things would result? For example, ...
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### Calculation of Terrestrial and Universal time scale difference, ΔT

$\Delta$T is defined as the 'measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of atomic time'. I tried to calculate $\Delta$T corresponding to ...
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### Discrepancy in Horizons planet ephemerides for equivalent times in TT and UTC in dates during 1969

I have recently been experimenting with JPL Horizons planet ephemerides web App, and found a puzzling discrepancy. Horizons allows to specify the time at which the planet ephemerides should be ...
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### Time in 0 gravity points

If being close to a supermassive body like a black hole makes time pass more slowly for us than for an observer from a point of view with a weaker gravitational field, if we get to be at a point in ...
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### How to determine sidereal time just looking at the picture?

I have this picture: I need to determine what sidereal time it is (in hours), or how long has it been since the upper culmination of the point of the vernal equinox. How can I do that?
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### Daytime length and sunrise / set cardinal directions

Let me illustrate the question with an example. On Feb 2nd 2022, the sun rises in Los Angeles at cardinal direction 110 and sets in direction 250 (according to timedate website). This means that the ...
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I wonder how to calculate the duration of the Sun crossing the line of the horizon on equinox on Pole. For equator I used this formula: $t = \frac{2 \times p}{\cos(\phi) \times u}$, where $u$ - is ...