Questions tagged [time]

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

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1answer
7k 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
157 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, ...
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3answers
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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 ...
7
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1answer
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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 ...
6
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2answers
218 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 ...
10
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3answers
15k 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
8k 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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7answers
4k 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 ...
1
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1answer
160 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. ...
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0answers
4k 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? ...
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2answers
2k 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 ...
4
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1answer
258 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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77 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 ...
4
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1answer
2k 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 ...
3
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1answer
133 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 ...
3
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3answers
221 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 ...
3
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1answer
107 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 "...
3
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2answers
297 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 ...
2
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1answer
273 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 ...
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0answers
53 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 "...
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2answers
875 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 ...
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1answer
67 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? ...
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1answer
68 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 ...
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1answer
104 views

How to calculate sunrise and sunset on on a given date and location?

I have a given date at any time in future or past eg: 2016-04-23 or 2022-05-12 Now given at any location(lat,lng). How do i find the time of sunrise or sunset.
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2answers
104 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 ...
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1answer
93 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 ...
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3answers
225 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 ...