Questions tagged [time]
Questions on time, the indefinite progress of events in the Universe.
224 questions
-1 votes
3 answers
651 views
Is universe without matter aging? [closed]
Cosmological time is equal to the proper time of hypothetical, immortal observer resting in CMB reference frame. If there is no matter, there is no observer with his proper time. I identify the ...
user52339
13 votes
3 answers
1k views
Since the universe is expanding and spacetime is a single fabric, does time also stretch along with the expansion of space?
I’m a student trying to understand the expansion of the universe and spacetime. I’ve read that the expansion means the space between two objects (like galaxies) increases over time. But since space ...
- 139
0 votes
1 answer
138 views
Deduction of the equation of time
I'm studying analemmas and I'm trying to find a simple but analytic deduction of the equation of time useful for any day of 2025. I would like to use the equation of center $\nu - M \approx c_1 \sin(M)...
- 11
3 votes
1 answer
852 views
How to calculate the time and date from the moon's position, phase and the orientation of the phase? (yes it is possible)
In short: How to calculate the time and date when you see the moon and its phase and orientation in the sky from earth during night? I would like to know the practical/actual calculations or formulas ...
- 49
5 votes
0 answers
88 views
Relating distance modulus to age of the universe without involving redshift
I'm trying to replicate some plots from Perlmutter (2003) in Physics Today, specifically Figures 3 and 4. The x-axis of Figure 3 and the y-axis of Figure 4 are redshift and the scale factor; the ...
- 51
17 votes
2 answers
3k views
First Day of Spring 2025---Why weren't Sunrise and Sunset Exactly Twelve Hours Apart? [duplicate]
March 20, 2025 is the first day of Spring this year in the northern hemisphere. Thus, on this day, night and day are both of equal length; and so, I would expect the times of sunrise and sunset to be (...
- 317
5 votes
2 answers
193 views
What are the chances that a star currently in the Sun's galactic neighbourhood will eventually revisit the Sun in the distant future?
There are thousands of known stars visible to the naked eye, and have been mapped in constellations for millennia. However, the stars that exist in our neighbourhood are currently there by chance, and ...
- 309
0 votes
1 answer
93 views
The time that a star is over the horizon
Could anyone please tell me about how can i calculate the time that a star is over the horizon, by it's hour angel when it's rising?
1 vote
1 answer
113 views
Sidereal Time, how to make your own observation similar to observatories?
When finding sidereal time, do I look for (perhaps 29 Piscium) in Pices to sweep past a vertical pole while viewing along the line of the N/S at the meridian? By naked eye, can I sight using the star ...
- 185
0 votes
0 answers
70 views
Egyptian Decan / Zodiac Star Constellations and measuring time/date at night?
I read that there are 36 stars 10 degrees apart in the celestial zodiac and am reading historically that these may have been used to tell time and date at night. Does anyone have any information on ...
- 185
8 votes
4 answers
3k views
UTC Time, navigation. Why are the time zones not following perfect meridian circles for longitude?
Why are these meridian lines for time not following straight up and down every n (15 deg) from the poles of earth? Does it have something to do with making time zones easier to understand by country? ...
- 185
1 vote
2 answers
106 views
Using astrophotography for positioning and timekeeping
I know plate solving is common method in astrophotography for determining where the telescope/camera is pointing at. My question is, can this data (series of plate-solved observations) be used to ...
- 11
-6 votes
1 answer
140 views
Ellipses as Planetary motion, What is the force causing the ellipses? [closed]
What is the exact force which causes Ellipses? And in what way does the empirical observation of the analemma, provide a support or a denial of ellipses? Several centuries ago, there had been a ...
4 votes
1 answer
459 views
Equation of Time (derivation Analemma)
In the document available here: http://info.ifpan.edu.pl/firststep/aw-works/fsII/mul/mueller.pdf I have been able so far to reproduce all missing details so far to the relation (33) of page 7: $$\sin ...
- 253
0 votes
0 answers
80 views
sidereal time calculation through centuries
I found this when I am studying ancient astronomical notes using Stellarium In 1770, the date when local sidereal time = local mean solar time (on the screenshot ...
- 311