# Questions tagged [time]

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

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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? ...
140 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?
72 views

### 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. ...
108 views

### 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 ...
73 views

### How can I calculate 27 tropical/solar year since my birthdate?

Let's say I was born on 27th November 1993 at 03:15:15 AM. Is there any way I can calculate exact 27 tropical cycle since my birthdate/birthtime. What date and what time it will be after exact 27 ...
72 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 ...
67 views

### Is there a way to know the time according the moon appearance?

Is it possible know the time according to the moon location? To look at the sky at night and then determine what the time is.
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### Can I use the Tokyo Skytree as clock?

After my observation today at the Skytree in Tokyo, I am wondering if this building could be used as a giant sundial? As an aside, it would be interesting to consider how the Skytree might be used ...
2k views

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### Does the Milky Way's movement through space have spacetime implications?

The way I usually see spacetime explained, it says that if you just sit still, you are moving at 0kph and so you are moving at maximum speed through time (ie. time for you is flowing at maximum speed)....
57 views

### Can you go blind if you look at the sunrise and sunset? [duplicate]

Everyine knows its dangerous to look directly at the sun, but can you look safely at sunrises and sunsets without going blind? Because the uv radiaton and the light is much weaker. Or is it still very ...
257 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 ...
216 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 ...
277 views

### calculating rising time from culmination time

I want to know how to calculate the rising time of a star, if culmination and setting times are available with me? Say Culmination time is 03:00 on 28-Nov-2018 and Setting time is 05:30 on 28-Nov-...
342 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....
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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 ...
68 views

### How does the geometry for constructing a declining vertical sundial work?

I understand how to construct South-facing vertical an horizontal sundials and the geometry behind their construction by projecting an equatorial sundial into a plane, but I can't quite get how to do ...
83 views

### Existence of a “reflecting” star [closed]

Wasn't even sure about what the title should be, but I had a thought and I wonder whether it may be possible or not. Is it possible that there is a star which acts as a mirror and reflects back the ...
299 views

### How could time only have started with the Big Bang?

I understand that before the Big Bang time (as well as dimensions) didn't exist. But how could this be? If there was no time then nothing could change, and so time itself couldn't come into existence.
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### Clarification on Greenwich Sidereal Time (GST)

I know that LST = HA + RA. And GST is LST + longitude(L). If a star culminates, it's HA=0. Therefore, GST = RA + L. Since RA and L are constants, GST must also be a constant. So, how can GST be ...
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### What's the time in which the sun rises? [duplicate]

What's the time the sun takes in order to rise? (The time in which the solar disk crosses the horizon) How does this time depend on local latitude and the sun's declination? Assume that we're at sea ...
78 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 ...
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### Is the Milankovitch calendar provably better than the Gregorian one considering a non-constant tropical year?

It's somewhat of a no-brainer the revised Julian calendar (attibuted to Milankovitch) is more accurate than the Gregorian one just because it started with a more accurate estimate of the duration of ...
610 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 ...