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 $c$ ...
- 2,632
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 ...
- 7,659
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 ...
- 129k
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 ...
- 7,659
48
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 ...
- 129k
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 ...
- 36.3k
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 ...
- 16.7k
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 ...
- 1,882
29
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 ...
- 36.3k
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 ...
- 2,263
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, ...
- 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 ...
- 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 ...
- 9,869
20
votes
Accepted
Always orient a sundial towards polar north?
A sundial needs to be aligned with the planet's axis of rotation, with the gnomon pointed in towards the nearest pole, for it to work correctly. The point of the shadow will then give you your Local ...
- 1,454
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 ...
- 739
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 ...
- 162k
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 ($\...
- 162k
16
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 ...
- 30.7k
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 ...
- 129k
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 ...
- 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 ...
- 249
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 ...
- 241
14
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 ...
- 918
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 ...
- 162k
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 ...
- 129k
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 ...
- 36.3k
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. ...
- 16.7k
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 ...
- 211
11
votes
Why are there not a whole number of solar days in a solar year?
Take a bike and a race track. Mark one spot on the side of one of your tires.
Now place the bike on the starting line, making sure the mark is precisely at the lowest point of the wheel (where it ...
- 465
11
votes
How did the ancient cultures determine that the year was actually a fraction of an extra day beyond 365 days?
For the most part, they didn't. Calendars have historically been a practical device. They were used to measure times of events, such as the planting and harvesting of crops. They only needed to be ...
- 605
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