# Questions tagged [time]

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

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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: ...
224 views

### 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....
1 vote
102 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 ...
1 vote
69 views

### 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
134 views

### 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. ...
1 vote
72 views

### 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 ...
43 views

### 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 ...
180 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 ...
89 views

### 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 ...
2k views

### Local mean time & Solar Noon

Is local mean time supposed to represent actual sun movement? Does 12pm on local mean time (not clock time), mean that the sun is on meridian or solar noon? I understand LMT means 1 degree for 4 ...
485 views

### 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 ...
1k 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 ...
1 vote
113 views

### 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 ...
6k views

### 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 ...
1 vote
5k 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? ...
1k views

### 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 ...
163 views

### 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 ...
1 vote
52 views

### 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 ...
553 views

### 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. ...
129 views

### 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
4k views

### Why does time pass faster in moon than the earth?

One of my friends said that, time passes faster in moon than the earth Why does time pass faster on the moon than on the earth?
97 views

### 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 ...
3k 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 ...
3k 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 it ...
126 views

### 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)?
283 views

### 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 ...
175 views

### 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 ...
184 views

### 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 ...
6k views

### 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 ...
445 views

### 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 ...
376 views

### How long was a lunar month in ancient times?

As the Moon is getting farther and farther from Earth, it's natural that the time it takes to circle around our planet is going to increase, thus the month will be longer. However, I read that the ...
726 views

### 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 ...
1 vote
201 views

### 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.
294 views

### 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 ...
1 vote
78 views

### 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 ...
146 views

### 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 (...
707 views

### 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 ...
143 views

### 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, ...
7k views

### 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(...
570 views

### 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?
38k 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 ...
369 views

### 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 ...
2k views

### Effect of the obliquity of the ecliptic / tilt of the Earth on the equation of time

In 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 ...
585 views

### Equation of time with arbitrary eccentricity and obliquity

I'm working on an on-line app to show the Equation of Time for arbitrary eccentricity and obliquity of the Earth's orbit. It looks good for small eccentricities and obliquities: but for bigger ones ...
267 views

### 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 ...
174 views

### 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, ...
144 views

### 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 ...
486 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 ...