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Questions tagged [time]

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

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12 votes
1 answer
11k 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 ...
user3574623's user avatar
9 votes
1 answer
921 views

How accurate is the "Equation of time" (mean time to actual solar time)? And how much can it vary from the average?

At Wikipedia, they give equal values (9.87 min) for all four extremes – both troughs and both crests – caused by the obliquity of the ecliptic as seen in the graph below, where the purple dashed line ...
George Lee's user avatar
7 votes
2 answers
490 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 ...
Nerrolken's user avatar
  • 637
3 votes
1 answer
357 views

So, what, exactly, is a tropical period?

From timezone.com's On the Units of Time Part III: The Year February 19, 2001, by Edward Hahn: Uh, Oh…He’s Going to Ask That Question Again… So, what, exactly, is a year? @PM2Ring's comment FWIW, ...
uhoh's user avatar
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8 votes
1 answer
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 ...
Mike Warren's user avatar
8 votes
6 answers
39k 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 ...
Deepeshkumar's user avatar
6 votes
3 answers
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 ...
emotionallyattached's user avatar
5 votes
2 answers
16k 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 ...
Carles Araguz's user avatar
2 votes
2 answers
860 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 ...
usernumber's user avatar
  • 17.6k
11 votes
7 answers
5k views

Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?

Background: I'm training to be a geography teacher. Currently I have practice lessons and I'll be discussing solar time and standard time with the class. Now I stumbled over an issue to which I could ...
mfran's user avatar
  • 139
9 votes
3 answers
16k 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 = ??? ...
wim's user avatar
  • 193
5 votes
2 answers
265 views

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?
2080's user avatar
  • 1,890
4 votes
1 answer
2k views

How much time passes in the outside universe when falling into a black hole?

If someone were to jump into a black hole, looking back at Earth, how far into the future would they see of Earth? As I understand it, time would dilate approaching infinity as you approach the event ...
Jonathan's user avatar
  • 4,381
1 vote
1 answer
474 views

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. ...
Suresh Chaudhary's user avatar
1 vote
0 answers
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? ...
Phiteros's user avatar
  • 3,166
29 votes
3 answers
9k views

If months are based on the moon, then why are the months longer in the Gregorian calendar than lunation?

What I have always thought / known is that: Days are based on the period of rotation of the earth. Months are based on the moon. Year is the time taken by the earth to make one revolution around the ...
lunisolar's user avatar
  • 391
22 votes
2 answers
7k views

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?
Zamicol's user avatar
  • 1,005
9 votes
2 answers
3k views

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 ...
astro's user avatar
  • 191
8 votes
2 answers
997 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, ...
hmakholm left over Monica's user avatar
7 votes
3 answers
11k 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 ...
Harry Weston's user avatar
6 votes
2 answers
284 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 ...
uhoh's user avatar
  • 30.7k
5 votes
1 answer
486 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 ...
Peter's user avatar
  • 1,033
5 votes
1 answer
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 ...
elika kohen's user avatar
5 votes
2 answers
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 ...
user avatar
4 votes
1 answer
305 views

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 ...
got trolled too much this week's user avatar
4 votes
2 answers
661 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. ...
Jagerber48's user avatar
4 votes
3 answers
266 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 ...
Darth Scitus's user avatar
4 votes
1 answer
466 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 ...
user8110's user avatar
3 votes
1 answer
437 views

Synodic Day and Sidereal Day

If given the sidereal day, is it possible to derive the synodic day of Earth? If a derivation is possible, could anyone illustrate it or point me in the right direction? Wikipedia's Synodic day begins:...
Shaun Ethan C. Phangcho's user avatar
3 votes
0 answers
101 views

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 ...
uhoh's user avatar
  • 30.7k
3 votes
1 answer
222 views

Validity of ephemeris time

In the wikipedia article for ephemeris time as first adopted in 1952, it does not explain clearly how or why it was superseded. For example, it reads: In 1976 the IAU resolved that the theoretical ...
Tyler Durden's user avatar
3 votes
2 answers
1k 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 ...
Gabriel Fair's user avatar
3 votes
1 answer
155 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 "...
Sir Cornflakes's user avatar
3 votes
1 answer
307 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 ...
Beijixing's user avatar
2 votes
1 answer
334 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 ...
David Vorick's user avatar
1 vote
1 answer
183 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? ...
Augunrik's user avatar
  • 113
1 vote
1 answer
104 views

How can I correct for transit time?

I know the exact time a radio telescope detected a transient event. I also know the exact location of the telescope, and the galactic coordinates (galactic longitude, latitude) and right ascension and ...
PerplexedDimension's user avatar
1 vote
2 answers
5k views

Is a year really 365.24 days, or is it 365.2564 days like I remember?

The NPR News item and podcast Spring Starts Today All Over America, Which Is Weird includes the following: But why isn't the time of the equinox the same each year? The short answer is that the time ...
uhoh's user avatar
  • 30.7k
1 vote
2 answers
834 views

Is the information about space we have old? [duplicate]

As we know light has a definite speed i.e. $c=299792458$ m/s so it takes some big time to travel from distant galaxies, right? So I always wonder about the big telescopes and big space companies ...
Gamin8ing's user avatar
1 vote
0 answers
97 views

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 "...
uhoh's user avatar
  • 30.7k
0 votes
2 answers
193 views

Timekeeping on other celestial bodies than Earth

I have looked for months for any site capable of calculating solar altitude w.r.t an observer located on the surface of a generic celestial bodies, but with no luck as of now. Even NASA does not ...
jumpjack's user avatar
  • 569
0 votes
1 answer
198 views

I'm 8 hours from Greenwich, why is my sidereal time 10 minutes different from theirs?

In order to write and run the script in this answer I typed "Greenwich sidereal time right now?" into google (or at least tried to) and got two numbers. Greenwich sidereal time was 09:51:07 ...
uhoh's user avatar
  • 30.7k
0 votes
3 answers
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 ...
user2682025's user avatar