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72 votes

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

"Accelerate instantly" would imply that a photon takes many different velocities at the same point in time. In fact, it would imply that a photon takes on every velocity between $0$ and $c$ ...
Nuclear Hoagie's user avatar
63 votes

How were sundials and moondials possible 800 years ago?

As @JohnHoltz points out in a comment, planting a stick in the ground or in a wall and watching where the shadow falls is something very easy; sundials have been known since prehistoric times. I’m not ...
Pierre Paquette's user avatar
53 votes
Accepted

Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?

The Earth takes 23 hours 56 minutes to rotate once. But that is not relevant to most people. Sure, the stars will be in the same position again after 23 hours 56 minutes, but the sun will not be in ...
James K's user avatar
  • 123k
50 votes

If months are based on the moon, then why are the months longer in the Gregorian calendar than lunation?

Ancient Egyptians and Mesopotamians came up with “administrative” calendars of 30 days, that were easier to calculate than “real” lunar months of sometimes 29, sometimes 30 days. At the end of the 12 ...
Pierre Paquette's user avatar
49 votes

Does the moon have days?

The answer depends on your definition of a day. If you define a day as we usually define it in the Earth (time between the Sun is at noon or average time between sunrises, a 24 hours day in Earth), ...
Pere's user avatar
  • 1,760
47 votes
Accepted

How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?

This is not so hard to do. All you really need is a bit of care, a few sticks and a convenient place to observe the sunrise or sunset. So note that there is a cycle of the year and you want to know ...
James K's user avatar
  • 123k
45 votes

How were sundials and moondials possible 800 years ago?

Those 2 wheels work like a perfect sundial. No they don't. They're flat. The timespan from one hour to the next on a flat sundial varies from season to season. At the same time that that temple was ...
David Hammen's user avatar
  • 34.1k
37 votes
Accepted

How many light seconds away is the JWST?

Almost 5.1 seconds, plus or minus around 1 second. Here's a daily plot (at midnight) of the light travel time from the JWST to the centre of the Earth, courtesy of JPL Horizons, using a script derived ...
PM 2Ring's user avatar
  • 14.8k
30 votes
Accepted

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

You make a great point. The reason behind the discrepancy between the dates is due to a complicated history behind it. The calendar is based on the calendar created by ancient Romans, which is based ...
Max0815's user avatar
  • 1,862
30 votes

Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?

We teach the students: Sidereal day: In 23 h 56 min the earth rotates 360° Solar day: In 24 h the earth rotates 361° You should not teach your students that. You should instead teach your students ...
David Hammen's user avatar
  • 34.1k
28 votes

Why are there not a whole number of solar days in a solar year?

if the sun appears to be in the exact point in the sky as that time the previous solar year, This statement means the sun is at (nearly) the same position relative to the stars that it was the ...
BowlOfRed's user avatar
  • 2,085
26 votes

How many light seconds away is the JWST?

According to NASA, the JWST is 1.5 million kilometers away, which is about 5 light seconds. For comparison, the distance to the Moon is about 1.3 light seconds, Mars ranges from about 3 to 22 minutes, ...
Zamicol's user avatar
  • 1,005
25 votes

How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?

It's easier than you think to notice the fraction... simply because the rounding error compounds over the years! After a good amount of years, you notice that solstices are drifting from their ...
Offirmo's user avatar
  • 351
22 votes

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

I am not sure this is a problem of visual communication. My incling would be to think this is a problem of language communication. The equations of relativity tell us that anything with zero rest-mass ...
AtmosphericPrisonEscape's user avatar
19 votes
Accepted

Does the moon have days?

Defining that a day is a rotation on the axis, 1 moon day is approximately equal to 27.3 Earth days. This link on moon rotation may interest you.
CipherBot's user avatar
  • 1,301
19 votes

Why are there not a whole number of solar days in a solar year?

Let‘s turn your question around: Why should they be in sync? The only mechanism that redistributes angular momentum between two celestial bodies are tidal forces (not considering general relativity ...
Grimaldi's user avatar
  • 704
18 votes

How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?

The ancient Egyptians appeared to know that the year was 365.25 days. This was established by observing on what day the heliacal rising of Sirius occurred (i.e. on what day that it first becomes ...
ProfRob's user avatar
  • 153k
17 votes

What are the stages in the life of a universe?

Yes there are. They are mainly based on what dominates the energy density of the universe at the time and they are known as epochs. Thus we have the inflationary epoch in the first tiny fraction ($\...
ProfRob's user avatar
  • 153k
16 votes

Why are there not a whole number of solar days in a solar year?

It isn't in the same place. We can measure "one year" starting at any point in time, suppose we start at sunrise on January 1st at some convenient location (eg London). We know where the sun ...
James K's user avatar
  • 123k
15 votes

How is time defined in astronomy?

$t$ signifies time; see the Wikipedia article for spacetime, and then the subsection for 4-vectors. The basics are pretty natural to understand. Suppose something happens, an event, like an apple ...
uhoh's user avatar
  • 30.5k
15 votes

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

You are looking for a way to visualize the fact that a photon is created traveling at the speed of light. Remember that a photon is actually a perturbation of the electromagnetic field. That field is ...
JohnHunt's user avatar
  • 816
14 votes
Accepted

Coordinate system for space

For locating objects in the sky, the horizontal and equatorial coordinate systems are commonly used. These systems describe the position of some object in the sky very well, but do not explain the ...
Tosic's user avatar
  • 1,681
14 votes

Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?

All of the other answers are fairly technical, but a decently simple logic chain forces time zones to be 360° / 24. Consider this: Since there are 24 hours in a day, it makes sense to divide the ...
bobsbeenjamin's user avatar
14 votes

If months are based on the moon, then why are the months longer in the Gregorian calendar than lunation?

Calendars can either be lunar or solar. A lunar calendar has months that match the phases of the moon but years that don't match the Earth's rotation around the Sun. It can't have both because the ...
Jack Aidley's user avatar
14 votes
Accepted

Depth of gravitational well within our local Virgo supercluster?

The size of the relative gravitational time dilation effect (when it is small), compared to a clock at infinity, is $\sim \Delta \phi/c^2$, where $\Delta \phi$ is the change in potential You can then ...
ProfRob's user avatar
  • 153k
13 votes
Accepted

Is time finite or infinite?

As far as we can tell, the four-dimensional space-time continuum is unbounded in the space directions (space is infinite) the time dimension seems to have a singularity (about 13.8 billion years ...
James K's user avatar
  • 123k
12 votes
Accepted

Julian Day Calculation

The formula in the wikipedia article explicitly uses integer division with round toward zero. Python's integer division uses round toward negative infinity (i.e., floor). The wikipedia article formula ...
David Hammen's user avatar
  • 34.1k
11 votes

Photon Paradox?

And that is why you don't do the calculations in a frame that is moving at lightspeed. If you have two observers that are moving relative to each other you can use the Lorentz transformation to ...
James K's user avatar
  • 123k
11 votes

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

Take a look at this diagram from Feynman's lectures at Caltech on angular momentum. Here, an atom with angular momentum $m=1$ starts out in an excited state on the left hand side of the diagram. ...
Connor Garcia's user avatar
  • 16.3k
11 votes

How were sundials and moondials possible 800 years ago?

They were possible millenia ago as all that was needed to set them out was observation and marking of hour angles. Equatorial sundials, i.e. a sundial whose plane is oriented parallel to the equator's ...
Trunk's user avatar
  • 211

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