Questions tagged [time]
Questions on time, the indefinite progress of events in the Universe.
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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 ...
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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 ...
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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 ...
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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 ...
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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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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 ...
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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 ...
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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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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 ...
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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 ...
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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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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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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 ...
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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:...
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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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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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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 ...
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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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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 ...
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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, ...
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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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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 ...
user45319
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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 ...
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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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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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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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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 "...
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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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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 ...
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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 ...
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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 ...
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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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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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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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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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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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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 ...
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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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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 ...
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