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71 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 $c$ ...
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51 votes
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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 ...
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50 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), ...
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48 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 ...
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28 votes
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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 ...
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28 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 ...
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22 votes
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When we see the Sun, do we actually see its past?

Yes, you are right. We don't only see the Sun 8 minutes in the past, we actually see the past of everything in space. We even see our closest companion, the Moon, 1 second in the past. The further an ...
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21 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 ...
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20 votes
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How can we avoid needing a leap year/second?

Leap years exist for two reasons: There are not an integer number of days in a year. People perceive a need to keep the seasons where they are on the calendar. Given the above, there is no way to ...
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20 votes
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Years, months, days, and ... weeks?

The synodic period of the moon is $29.53$ days, a little shorter than a calendar month, which is on average about $30.4$ days. This is slightly longer than its orbital period, but corresponds to the ...
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20 votes
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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.
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  • 1,261
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 ($\...
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  • 115k
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 ...
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14 votes
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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 ...
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  • 1,616
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 ...
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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 ...
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14 votes
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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 ...
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  • 115k
13 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 ...
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12 votes
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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 ...
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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 ...
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10 votes
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Why is JD 2451545.0 = January 1, 2000, Noon instead of JD 2451558.0?

This Wikipedia article states (correctly) that "The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Greenwich Mean Time, with ...
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  • 604
10 votes
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Local Sidereal Time

LST = 100.46 + 0.985647 * d + long + 15*UT They don't explain what the two constants are (100.46 and 0.985647), could anyone explain what those constants are ...
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  • 28.1k
10 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. ...
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10 votes
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Is the information about space we have old?

When I look at my clock (which is 3 metres from my chair) I see how it was about 10 nanoseconds ago. I don't see the clock "now", but always a little time in the past. When you look at the ...
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  • 88.7k
9 votes

Why the continued obsession in measuring the year as a multiple of days?

Why all the stunts with leap days and leap seconds, etc... when we now have atomic clocks? Science exists to serve mankind, not to rule it. Calendars were one of the first concepts developed by ...
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9 votes
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How is the Gould Belt younger than the sun?

Everything is in motion in our Galaxy. The Sun has executed some 20 laps of the Galaxy since it was born and may have migrated inwards or outwards to some extent. The Sun's location has nothing to do ...
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9 votes

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

Here’s a slightly different way to think about it that might be helpful to you and/or the students. To answer the question “how long does it take the Earth to rotate once on its axis?” you have to ...
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  • 4,259
9 votes

Rotation of far away and near by galaxies

It depends a bit on what you means by "far away" and "the same", but: Galaxy formation Galaxies form from collapsing and colliding clouds of gas and dark matter in the early ...
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  • 32.3k
8 votes

Local mean time & Solar Noon

Local Mean time is based on the Average position of the sun. The actual sun might not be on the meridian at noon LMT but on average. Some times the sun moves faster and sometimes slower (for example ...
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7 votes
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When will a day on Earth & Mars be the same length?

Since the rate of lengthening of the day on Mars is three orders of magnitude less than that of the Earth for a first go at calculating how long it will be before the day lengths are equal we can ...
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