Questions tagged [time]

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

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25
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3answers
5k 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 ...
25
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1answer
5k views

What's the reason that we have a different number of days each month?

It always was interesting for me to understand the answer for the question: What's the reason that we have a different number of days each month? If the month is fixed on the time that the moon ...
17
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3answers
7k views

Does the moon have days?

As like Earth, does the moon have a day (24 hours). If it has, how many hours are in a moon day?.
13
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4answers
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How can we avoid needing a leap year/second?

Given the Earth's current speed around the sun and current rate & axis of rotation, what is the best way to keep time to avoid a leap year? How many hours should we have in the day and days in a ...
11
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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 ...
10
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3answers
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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 = ??? ...
10
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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 ...
9
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3answers
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How is time defined in astronomy?

I was amazed when my teacher told me that in space, time is define as a coordinate. It means we can define something with a four coordinate system with "negative time" $-t$ as easily as we can say $-...
9
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3answers
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Coordinate system for space [closed]

I am high school student, I have doubt if I have to locate a planet or any star in space, how do I locate it, because we have no perfect coordinate system, I have read in some books, that time is ...
8
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1answer
1k views

What are the stages in the life of a universe?

Geological periods describe varying phases in the last few hundred million years on earth. Are there any names for the periods on a timescale 1-2 orders of magnitude greater, that is, for the stages ...
8
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1answer
321 views

Why aren't GPS clocks sufficient to synchronize VLBI observations?

In very long baseline interferometry, it is typical to use a hydrogen maser atomic clock to sample data. However, given how expensive these atomic clocks are, it seems surprising that more ...
8
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2answers
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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 ...
8
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1answer
474 views

Universe is expanding, why doesn't time expand with it?

Reading this response When the universe expands does it create new space, matter, or something else? I understand Universe is not expanding like if it is stretching its space, but it is actually ...
7
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2answers
464 views

Rotation of far away and near by galaxies

Do the far galaxies rotate the same as does the milky way and the near by galaxies?
7
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3answers
739 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, ...
7
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2answers
169 views

September 26 twelve hours sunrise to sunset?

I noticed in today's newspaper (Boston Globe, September 26) that sunrise and sunset were both at 6:25. That's the twelve hour day I'd have expected at the equinox. (Please retag if there's a better ...
7
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1answer
1k views

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 ...
7
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1answer
161 views

What are the problems that need to be taken in consideration when trying to find planetary position by using jpl ephemerides?

I'm currently trying to determine the position of the sun and the moon from jpl ephemeris DE200 which is referred to the dynamical equator and equinox of 2000 and uses tdb time. I am using the j2000 ...
6
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3answers
24k 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 ...
6
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3answers
939 views

Julian Day Calculation

I'm trying to calculate the Julian Day, given a UTC year, month and date in the Gregorian calendar. I tried using the formula on Wikipedia, but that doesn't work. Consider 2010-01-31 and 2010-02-01. ...
6
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2answers
219 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 ...
6
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3answers
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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 ...
6
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2answers
302 views

How do we measure the age of the universe?

As mentioned in wiki/Age_of_the_universe, The current measurement of the age of the universe is around 13.8 billion years (as of 2015) – 13.799±0.021 billion years When my friend who is not from ...
5
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3answers
616 views

What does BJD-2450000 mean?

In different papers I've encountered time indications in BJD-2450000. I know that BJD stands for Barycentric Julian Date, but I don't understand the meaning of the suffix -2450000. This is an example ...
5
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2answers
352 views

Why isn't the day backwards between leap years?

So every four years, we add an extra day. A leap year. This is because a year is actually $365_{1/4}$ days long. But what happens between leap years? After two years, it would get dark at 8:00 AM ...
5
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1answer
2k 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 ...
5
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2answers
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“Official” time for other planets

I want to make clock that shows current time on different solar bodies. For that I need to know how to count time on those planets. Is there any official definition of times on other planets in our ...
5
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4answers
722 views

Is sundial time entirely dependent on solar azimuth?

I've visited several "how sundials work" sites and can't seem to get a clear answer to this: is "sundial time" just a linear function of solar azimuth? More specifically: When the sun is due south ...
5
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1answer
342 views

Is there a variation of ISO 8601 for julian calendar dates?

If I want to write a date in the Gregorian calendar, I can write it in the ISO 8601 format; today would be 2016-02-20. The advantage is that there are many parsers available to let computers read in ...
5
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1answer
758 views

How to calculate geocentric conjunction time and moon altitude at a given time

Umm al-Qura calendar of Saudi Arabia determine the new moon if these criteria meet at 29th day: The geocentric conjunction occurs before sunset. The moon sets after the sun. I've got the formula to ...
5
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0answers
93 views

Why does “longitude by chronometer” require first calculating latitude?

I'm studying the history of longitude and Greenwich Time, and I'm currently confused about the "longitude by chronometer" technique. Conceptually, the method seems extremely simple. With a ...
4
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2answers
3k views

Why is JD 2451545.0 = January 1, 2000, Noon instead of JD 2451558.0?

I know that JD0.0 is Jan. 1st 4713 BC at Noon, a year in the Julian calendar is 365.25 days and that the number after 'JD' is the number of days so... I want to calculate the number of days between ...
4
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4answers
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Photon Paradox?

Imagine a photon reaching the Hubble Space Telescope today had originally been emitted from a star in the early universe 13 billion years ago. Einstein’s Special Relativity tells us, traveling at the ...
4
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1answer
156 views

How is the Gould Belt younger than the sun?

Our sun is located in the Gould Belt, a group of stars which is thought to be 30 - 50 million years old. However, the sun formed 4.6 billion years ago. Did the belt form around us?
4
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1answer
406 views

Sorting out Julian Day, Julian Date, Julian Day number, Julian Day Calendar, and Julian Day Table

In this answer I mention day number which is 1 on the first day of each calendar year (January 1) and increments to 365 or 366 on December 31 of that year. There was an edit proposed, which included ...
4
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4answers
262 views

What is this “time travel” these popular blogs refer to?

I am getting confused by some "popular science" about time travel, such as or "Time Travel Is Real. Here Are the People and Spacecraft Who Have Done It": It is indeed an interesting property of ...
4
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1answer
700 views

Does Time Exist for Photons? [closed]

If time slows down the closer you get to the speed of light, does it follow that time does not exist for photons. Do photons - even if they travel across billions of light years of space - arrive at ...
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 ...
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 ...
4
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1answer
172 views

Do today's professional astronomers use earth rotation angle or sidereal time?

I have read that "earth rotation angle" is the modern replacement for "Greenwich sidereal time". Is it actually the case that modern professional astronomers are using earth rotation angle? Either way,...
3
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2answers
299 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 ...
3
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1answer
933 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 ...
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
595 views

When will a day on Earth & Mars be the same length?

I read on Wikipedia that: As on Earth, the period of rotation of Mars (the length of its day) is slowing down. However, this effect is three orders of magnitude smaller than on Earth because the ...
3
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1answer
158 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, ...
3
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2answers
129 views

What are the Time Zones on Mercury?

I normally post on worldbuilding, but this question is merely about facts of a planets orbit and spin. I am trying to figure out how time periods work on Mercury to possibly make a worldbuilding guide ...
3
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2answers
85 views

Are there predictions of the distribution of matter at the end of time

Are there any theoretical predictions or "educated speculation" by cosmologists on how matter will be distributed at very, very distant times in the future? For example, will matter be: agglutinated ...
3
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1answer
106 views

How often and over what period is Earth’s rotation averaged to compute UT1?

I understand that UT1 (and for that matter UT0, UT2, etc.) are based on averages of actual earth rotation, and serve as a form of mean solar time. However it’s not clear to me when these averages are ...
3
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1answer
602 views

In the calculation of GMST, what are these constants? [duplicate]

GMST = 6.697374558 + 0.06570982441908 D0 + 1.00273790935 H + 0.000026 T2 From the GMST calculation on the USNO website (and many other places) we encounter these constants: 6.697374558 0....
3
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1answer
137 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 ...