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help with something that has puzzled me for years

seanplod

By seanplod
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seanplod

seanplod

Posted
Posted

Ok,

I totally understand about why we have seasons due to the tilt in the earths axis, but what I cant get my head around is this ; Imagine the earth with the northern hemisphere in the middle of winter top points away from the sun think of great Britain facing the sun. Ok now same thing only this time mid summer with the Britain in the same place which will now be in darkness so why is summer not 12 hours different from winter ????????????? Am I being stupid ? is just due to the rotation of the earth if it is that simple then i still cant get my head around this conundrum 

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AstroJon

AstroJon

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Posted

I'm not sure I understand the question but I'll try to explain a little.

In our (Britain) winter, the Earth's axis is pointing away from the sun. Imagine therefore that as the Earth rotates on it's axis, we spend more time with the sun behind us, i.e. night, than in front of us (day).

As for why it is winter when the axis is pointing away from the Sun, the Earth's surface is further away from perpendicular to the Sun than in summer so the same amount of incoming photons are spread over a wider area.

Does this help?

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gkec

gkec

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The time it takes the earth to complete a whole revolution is not quite the same as the length of a day. The length of a day is defined by the relation to the sun not the exact rotation of the Earth. If you look at the stars this indeed does happen as you see different stars in winter than in summer.

The rotation period is 23 hours and 56 minutes.  The remaining 4 minutes being accounted for by the orbit around the Sun.

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ronin

ronin

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Because a day of 24 hours is not the correct length.

The earth rotates at a sidereal rate and if we had a 24 hour siderial rate then 6 months later 12:00 midnight would see us looking at what is in effect a midday sun.

Sidereal rate is about 4 minutes different each day.

So as we rotate out clock time gets 4 minutes out of step per day. So 6 months later it is 732 minutes "wrong" or 12.2 hours out.

So insteead of "Midnight" having us pointed at the sun we are 12(ish) ours out, so it is or appears to be midnight (almost).

In effect Midnight as per our clocks has/is "conveniently" compensated.

Now this may be an expected effect, say this as it seems a bit too convenient.

The other bits are why we have a leap year efvery 4 and not a leap year every 100 unless it is a 1000 multiple - well something like that.

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wxsatuser

wxsatuser

Posted
Posted

Ok,

I totally understand about why we have seasons due to the tilt in the earths axis, but what I cant get my head around is this ; Imagine the earth with the northern hemisphere in the middle of winter top points away from the sun think of great Britain facing the sun. Ok now same thing only this time mid summer with the Britain in the same place which will now be in darkness so why is summer not 12 hours different from winter ????????????? Am I being stupid ? is just due to the rotation of the earth if it is that simple then i still cant get my head around this conundrum 

Because of Solar time.

The day is based on the position of the Sun in the sky.

Basically the Sun does 360 degrees every day and each day when it reaches

it's highest in the sky it is local apparent noon.......12.00 local apparent time.

http://en.wikipedia.org/wiki/Solar_time#Mean_solar_time

Try this simulator as well.

http://astro.unl.edu/classaction/animations/coordsmotion/siderealSolarTime.html

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JamesF

JamesF

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Welcome to SGL.

gkec and ronin have the answer, if I've understood the question correctly.

An "earth day" is measured by the position of the Sun.  The Earth's movement around its orbit means that in one day the planet actually rotates by slightly more than 360 degrees for the Sun to reach the same place in the sky.  The time taken to rotate exactly 360 degrees is a "sidereal day".  During the six months it takes for the Earth to reach the opposite side of its orbit this "slightly more than 360 degrees" rotation is enough to account for that apparent twelve hour difference that's confusing you.

A sidereal day can be thought of as the time it takes the Earth to return to the same position relative to the background of stars in the sky, as they're sufficiently distant that the relative motion between us is not as apparent.

James

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JamesF

JamesF

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And to further amuse, consider this...

At perihelion (the Earth's closest approach to the Sun), the Earth is five million kilometres closer than at aphelion.  Yet our northern hemisphere summer occurs when the planet is furthest away from the Sun and winter when we are closest.  So that little change in day length and angle of incidence of sunlight made by the 24-ish degree inclination of the Earth's axis of rotation is sufficient to totally dwarf the effect of moving the planet almost four hundred times its diameter nearer or further away from the Sun.

James

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