How Many hour are there in a day?

Answer 1

Now that you've seen all the somewhat derisive and cocksure answers, here is one that takes your question seriously.

It all depends on what you mean by the word "day."

1. The "mean solar day" (the "solar day" itself varies throughout the year) is defined as the average length of time throughout the year for the Sun to go from being "overhead" (really at its highest apparent elevation) to "overhead" again the next "day."

From ancient times our timekeeping was geared to the Sun (e.g. by shadow casting, sundials etc.) That kind of day did change its length a bit. Consequently, during the complete year the Sun would have become up to +/- 20 mins off from time kept by a modern clock. (A correction diagram, called the "analema," a kind of lop-sided figure of 8, geared to the months or signs of the Zodiac, is often inscribed on large sundials. You'll sometimes see it also, mysteriously and unexplained, on large maps or globes of the Earth; a related concept is "the Equation of Time.")

Since our modern clocks run at a very constant rate, the 24 hours that they register are geared to the iidealized "mean solar day." If you could make accurate measurements of the Sun's position in the sky, you'd find it would run fast or slow by +/- 20 minutes, depending on the time in the year.

2. The "sidereal day." This is a term used by astronomers. It's the time taken for the Earth to make one rotation with respect to the stars. Because of Earth orbiting the Sun, it works out that this sidereal day is a bit shorter than 24 hours. It's approximately:

23 hours, 56 minutes, and 4.1 seconds.

The "missing bit" of

~ 3 mins, 55.9 secs

is because as the Earth moves on in its orbit around the Sun, when it's rotated once with respect to the stars, it hasn't yet "caught up" with the direction to the Sun. It takes that extra ~ 3 mins, 55.9 secs to "catch up with the Sun."

Because of the difference between these two "days," the same stars "rise" and "set" ~ 3 mins, 55.9 secs earlier on each successive day in the year, finally coming back to appearing or disappearing at the same time only after a full year has gone by. This is connected with the next point.

The Earth's orbital motion is involved with something that often puzzles beginning students of astronomy. If you ask someone on Earth how many times does the Earth rotate with respect to the Sun throughout the year (equivalent to how many days there are in the year), if they're educated they'll say "365.25 times." ***(But seepostscript, below.) However, if you ask an observer out in space the same question, they'd answer "366.25 times." (You may need to draw a diagram to convince yourself of this.)

The "missing bit" above (~ 3 mins, 55.9 secs) is in fact 1 year divided by 366.25 (It's tempting to think it must be divided by 365.25, the number of daily rotations you're so familiar with; but a good diagram and careful thought should convince you it's 366.25. (Try it on your calculator!)

A "back of the envelope" check that the amount "missing" should be of the order of 4 minutes is this:

Approximate the relevant divisor by 360. Take the one differing rotation during one year as 24 hours. Then dividing that up equally, the "missing amount" per day would be 24 x 60 minutes divided by 360. So, the missing bit per revised day should be about:

24 x 60 mins / 360 = 24/6 minutes = 4 minutes.

That is cetainly of the right order.

(Scientists routinely do such calculations to see whether their understanding is on the right soccer field; I'm sorry, "in the right ball park" for American readers.)

I hope that this serious response commends itself to you.

Live long and prosper.

*** Postscript: The "365.25" is itself an approximation; the length of the year is really 365.242... days. (To give more figures would take us into further complicated territory of --- and the terrors of fathoming out --- the differences between the "tropical year," the "mean vernal year," not to mention other possible "years.") The fact that the year is slighty less than 365.25 days is why the usual "4th year rule" for leap years can'r work in the long run. Two adjustments, however, keep our calendar in phase with the seasons for more than 3,300 years:

Make "century" years (years ending in "00") NOT leap years, UNLESS they're divisible by 400.

I'm afraid that arnpu, below, got that mixed up. Check any calendar for the year 2000; it WAS a leap year, because 2000 IS divisible by 400.

Answer 2

24

Answer 3

24

Answer 4

24

Answer 5

24

Answer 6

24

Answer 7

24

Answer 8

24

Answer 9

24

Answer 10

24

Answer 11

24 hours