# Tag Info

70

"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 $c$ simultaneously, but that clearly makes no sense at all - a particle cannot have many instantaneous velocities simultaneously. When a photon is created, it is ...

51

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 the same position. It is far more important, for most people, to measure the time from noon to noon. And the average time from noon to noon is 24 hours. This is ...

50

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), the length of a day in the Moon is the synodic period of the Moon and it takes 29.530589 days (29 d 12 h 44 min 2.9 s). Anyway, if you define a day as rotation ...

48

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 months, they added five days of religious observances, sometimes positive, sometimes negative. When Julius Caesar conquered Egypt, he knew about the very ...

29

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 on one Moon cycle. One lunar cycle is 29.53 days. www.universetoday.com/20620/lunar-year/ which does not evenly divide into the 365.25 days of ...

28

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 that it takes the Earth 23 hours and 56 minutes to rotate 360° with respect to the remote stars. So why do we use a 24 hour day? The reason we use a 24 hour day ...

22

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 object is from us the longer its light takes to reach us since the speed of light is finite and distance in space are really big.

21

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 can only ever exist at the speed of light. So photons don't really accelerate, it is more a fundamental property of their existence to travel at c.

20

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 avoid leap years, or something similar. Defining the calendar year as being a fixed number of days (e.g., 365 days) would result in the seasons shifting by one ...

20

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 periodic visual appearance of the moon as viewed from Earth. I mention this to make it clear that we should be forgiving of a little imprecision. Conventionally,...

20

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.

17

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 ($\sim 10^{-32}$) of a second, when the energy density was dominated by an inflationary field. Then we are in the electroweak epoch, when the weak nuclear and ...

15

$t$ 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 falling off of a tree. In order to tell someone else about it you need the three space coordinates $x, y, z$ and the time coordinate $t$. Without all four, you ...

14

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 position of the object in space (if you know the distance you know "where" the object is, but this is relative to the equatorial/horizontal plane and is tricky to ...

14

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 Earth into 24 time zones. There are 360° in a circle, because that's how degrees are defined. To completely cover the circumference of the Earth, you must account ...

14

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 everywhere at once. It is part of the fundamental construct of the Universe and it has, essentially, always been so. An approximation of photon propagation ...

13

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 moon doesn't pass through an integer number of whole cycles in the year. The most common example of this kind of calendar is the Islamic Calendar in which dates ...

12

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 repeatedly uses (m-14)/12. This evaluates to -1 for months 1 and 2 (January and February), zero otherwise. You can use the wikipedia article formula in python3 ...

11

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 change between their frames of reference. But if one of the observers is a photon the lorentz transformation becomes singular, because $\gamma$ is infinite. Simply, ...

10

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 Julian day number 0 assigned to the day starting at noon on January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian ...

10

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 and how they were calculated in the first place please? There are three constants there, 100.46, 0.985647, and 15. The value of 100.46 degrees is the value needed to make the expression yield ...

10

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. Then it moves to a ground state as it emits a photon, traveling at $c$. Angular momentum is conserved, so the photon now has an angular momentum of 1. As an ...

10

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 sun, you see how it looked 8½ minutes ago. When you look at GN-z11 you see how it looked 13.4 billion years ago. That is a fundamental fact about our universe. ...

9

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 humankind, arguably predating writing. Timekeeping is even more important to modern society than it was to the ancients. You're not going to get rid of calendars. ...

9

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 with the Gould belt or vice versa. The Gould belt stars formed just 30 million years or so ago. The position of the Sun relative to the Gould belt is a ...

9

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 first answer “rotate with respect to what reference point?” If you ask how long it takes to rotate enough to bring a given star back to the same position, that is ...

9

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 Universe. The first structures began to form a few hundred million years after the Big Bang, with masses of the order of $10^5$ Solar masses (e.g. Mo et al. 2010). As ...

7

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 ignore the change in day length on Mars. The difference in day lengths is $\approx 40$ minutes. The Earths day lengthens by $\approx 1.7$ ms/100 years. Therefore ...

7

Analemma is a diagram showing the deviation of the Sun from its mean motion in the sky, as viewed from a fixed location on the Earth. Note that it says nothing about time- it can be any fixed time. Analemmas created at different times of the day have slightly different shapes. For example, see the solar analemmas taken at the same place at different times. ...

7

Another valid definition is the time between earthrises instead of sunrises, since the moon is in orbit around earth. In that case, there is no direct concept of a day, because the moon is tidally locked to earth.

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