Astronomy Stack Exchange
Astronomy Stack Exchange

Questions tagged [time]

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

205 questions
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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 is what is caused by the obliquity of the…
George Lee
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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, when I tried to locate an actual definition of UT1,…
hmakholm left over Monica
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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 one)
Ethan Bolker
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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. These dates are exactly one day apart, but…
vcapra1
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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 shorter than the solar day, for a given calendar…
Peter
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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 understood, both TT and TAI are continuous time…
Seninha
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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 this kind of date. But what if I want to write a…
Gerard Ashton
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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, what is the reason for using the one they use?
Gerard Ashton
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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 Jan1.2000AD and Jan1.4713BC and I do $$ \text{# of…
zeitoon
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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 earth is the same as the astronomical time? As…
Beijixing
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Converting UTC (10 digit) to LST (Local Sidereal Time)

I am an astro-particle physicist looking for some simulation based on the experiment observation. The simulation dataset contains 10 digit Epoch dataset for UTC time, however, I need to convert this into…
Kyle
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What kind of beast is Daylight Savings Time and Standard Time?

I understand that UTC, for example, is a "time system" and that there are regions of the globe that are divided into "time zones" (offsets from UTC); but what class of thing is, for example, Daylight Savings Time. It's not a "time" of course, nor,…
orome
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Equation of time with arbitrary eccentricity and obliquity

I'm working on an on-line app to show the Equation of Time for arbitrary eccentricity and obliquity of the Earth's orbit. It looks good for small eccentricities and obliquities: but for bigger ones the components of the Equation (the so called…
Luis López
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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 basis for its current (1952) standard of Ephemeris…
Tyler Durden
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How to convert sidereal time to local time?

I came across something that won't let me go, but no matter how much I calculate (which I do not like), it doesn't seem to work. Also: I've used so much apps and links already to convert, but it doesn't work in that direction. You see, local time…
Petra Stienaers
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