12hr days and 12hr nights?

Question

where on Earth experience 12hrs of daylight and 12hrs of night every day of the year?
How many times does the Sun pass directly overhead at noon on the equator during the course of a year?

Answer 1

Because the Earth's rotation axis is tilted relative to the plane of its orbit, the declination of the sun appears to cycle over the course of a year.

The Sun's declination is zero when it crosses the equator: spring equinox and fall equinox.

Maximum North declination is on June solstice (summer solstice in northern hemisphere).

Maximum South declination is on December solstice.

The Sun's maximum declination (North or South) is 23˚26.7' (90˚ minus the angle the Earth's axis of rotation makes with the plane of Earth's orbit around the sun).

Latitudes on Earth are based on the declination that passes directly overhead. Therefore, if you live between latitudes 23˚26.7' North and 23˚26.7'S, then the Sun will pass directly overhead on the day that its declination is equal to your latitude.

If your latitude is exactly 23˚26.7' (N or S), this occurs once a year (on the day of the appropriate solstice). For any other latitude in between, this occurs twice a year, once when the Sun's declination increases towards its maximum, once when it comes back.

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In theory, the duration of daylight is found through the equation:

Cos (D/2) = - tan(LAT)*tan(DEC)

Where D/2 is the polar angle of the Sun at sunrise or sunset, LAT is the latitude of the observer and DEC is the declination of the Sun.

Once you have found D in degrees, divide by 15 degrees per hour to convert in time units.

The day last 12 hours whenever D is 180 degrees (i.e., when D/2 is 90 degrees).

If D/2 = 90˚, then Cos(D/2) = 0

Therefore, either your latitude must be zero (equator) or the Sun's declination must be zero (day of equinox).

This is based on the Sun's centre being on the theoretical horizon. In practice, air refraction raises the image of the Sun and makes the day longer; also, people consider night to commence after the entire sun (not just its centre) is below the horizon. Therefore, your milleage may vary.

Answer 2

Excluding the effect of twilight, the only times this will happen is on the equinox, when the sun is midway in its travel north and south, when the axis forms an isosoles triangle with the center of the sun, i.e., the poles are equally distant from the center point of the sun. At that point, the day will be the closest to 12 hours at the equater. Since the year is not exactly 365 days, the time for this point changes from year to year by about 8 hours for 3 years and then 24 hours the other way during leap year. On any other day, the day is either longer or shorter, changing about 3-4 minutes per day. The 12 hour point travels north and south of the equater along with the sun. Only on the equinox will the sun be directly overhead at local noon at the equater. On any other day, the sun will be at more or less than exactly overhead (altitude of 90 degrees) at local noon. If you take a sighting on the sun at local noon every day and plot the position, you will see the sun traces a lopsided figure-8 because of the tilt of the Earth and the excentricity of the orbit. Only at the equinox, the center of the figure-8 where the path crosses center, is the day and night near equal lengths at the equater. The figure-8 is lopsided because the excentricity of the orbit is not in sync with the axis tilt, in other words, when the Earth is at our closest point of approach to the sun, the north pole is not closest to the sun. I hope I'm remembering all of this correctly from a class in my distant, distant past...

A nautical almanac used by navagators at sea contains tables of sun altitude at local noon for determining latitude north or south of the equater. Longitude is determined by measuring the actual point of local noon and how much it differs from a reference point, which simply takes an accurate timepiece which knows when local noon is at the reference point, using the equater like a hour hand of a 24 hour clock to determine east/west position from the reference point.

This link is to an example of the analemma, the path the sun takes through the sky over the period of a year from the Astronomy Picture of the Day site.

Answer 3

What do you mean by "daylight" and "night"? What about twilight?

What do you mean by "noon"? 12:00 on the clock? If so, I would say that the Sun is directly overhead at the equator only for an instant, twice a year; and only at one place on the equator each time. And that instant is not likely to be exactly noon on the clocks being used at that place.

Answer 4

fortunate you my hubby works 80 4 to ninety six hours a week. 5 12 hour nighttime shift and the two sat nighttime 12 hour or sat amazing via till monday morning a 24 hour shift, involved in the minimum salary. You sign an opting out style, something to do with The working cases Directive. He generally takes a million saturday nighttime a month off.

Answer 5

Do your own homework. If you do the reading and still don't understand, then ask a question about what you don't understand.

Answer 6

the equator at the solstices

Answer 7

wiki "equator" and find your answers.


Done.