Questions tagged [time]

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

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31 votes
6 answers
10k views

Why does light accelerate instantaneously to c, while no other phenomena do it? [closed]

In physics, it always takes some time for a particle to move from rest to some speed. However, photons (light particles) accelerate instantly from zero to c. How? (A visualization would be helpful.)
AnnexRemotelearn's user avatar
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
25 votes
1 answer
7k 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 ...
Reckless Glacier's user avatar
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
17 votes
3 answers
8k 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?.
RANSARA009's user avatar
16 votes
12 answers
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 ...
Jimmy Jam's user avatar
  • 169
13 votes
4 answers
5k views

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 ...
TheSmallestOne's user avatar
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
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
11 votes
1 answer
1k views

Depth of gravitational well within our local Virgo supercluster?

For simplicity we could imagine standing at the north pole so we do not have to care about how fast we are moving in relation to the gravitational field of the earth. Now calculating how much faster ...
Agerhell's user avatar
  • 457
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
9 votes
3 answers
2k views

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 $-...
आर्यभट्ट's user avatar
9 votes
3 answers
2k views

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 ...
आर्यभट्ट's user avatar
9 votes
1 answer
847 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
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
6 answers
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 ...
Deepeshkumar's user avatar
8 votes
1 answer
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 ...
stevec's user avatar
  • 181
8 votes
2 answers
984 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
8 votes
1 answer
513 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 ...
Hans Gaensbauer's user avatar
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
2 answers
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....
Rabbi Kaii's user avatar
8 votes
1 answer
749 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 ...
Giuseppe Bertone's user avatar
7 votes
2 answers
486 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?
nir's user avatar
  • 171
7 votes
2 answers
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 ...
Nerrolken's user avatar
  • 637
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
7 votes
2 answers
207 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 ...
Ethan Bolker's user avatar
7 votes
2 answers
759 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 ...
Sazzad Hissain Khan's user avatar
7 votes
1 answer
232 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 ...
Alexandru Lapusneanu's user avatar
6 votes
3 answers
2k 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. ...
vcapra1's user avatar
  • 295
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
6 votes
1 answer
368 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 ...
thevikas's user avatar
  • 171
6 votes
1 answer
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?
Peter's user avatar
  • 1,033
6 votes
1 answer
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 ...
Jagerber48's user avatar
6 votes
4 answers
914 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 ...
user avatar
6 votes
2 answers
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 ...
uhoh's user avatar
  • 30.8k
5 votes
3 answers
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 ...
Ed_Gravy's user avatar
  • 317
5 votes
4 answers
2k views

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 ...
Treeves3's user avatar
5 votes
3 answers
2k 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 ...
Laila's user avatar
  • 97
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
5 votes
2 answers
603 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 ...
uthbees's user avatar
  • 53
5 votes
1 answer
1k 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 ...
uhoh's user avatar
  • 30.8k
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
5 votes
1 answer
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 ...
Seninha's user avatar
  • 181
5 votes
2 answers
2k views

"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 ...
lsrom's user avatar
  • 163
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
1 answer
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 ...
Peter's user avatar
  • 1,033
5 votes
1 answer
220 views

Discrepancy in Horizons planet ephemerides for equivalent times in TT and UTC in dates during 1969

I have recently been experimenting with JPL Horizons planet ephemerides web App, and found a puzzling discrepancy. Horizons allows to specify the time at which the planet ephemerides should be ...
Rafa's user avatar
  • 351
5 votes
2 answers
251 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
  • 1,888
5 votes
1 answer
473 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 ...
Gerard Ashton's user avatar
5 votes
1 answer
871 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 ...
Josh Haberman's user avatar

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