Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: guest39538 on 01/03/2016 11:29:01
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oh the winter is cold and has 12 hrs or darkness, oh how the warm summer as about 5 hrs of darkness, how can this be without spinning faster?
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No, we don't rotate faster, it is all to do with the inclination of the earth as it goes round the sun.
Again Wiki has good explanations and better diagrams that we could draw here.
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The length of the day is the same (almost 24 hours) throughout the year. The proportion of the day that is light vs dark is a function of the latitude of the observer, and the angle between the Earth's axis of rotation and the plane of the orbit.
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oh the winter is cold and has 12 hrs or darkness, oh how the warm summer as about 5 hrs of darkness, how can this be without spinning faster?
People in the arctic circle see this in action:
- In mid-Summer, the Sun does not set - it takes 24 hours for the Sun to complete a revolution around the horizon*
- In mid-Winter, the Sun does not rise - it takes 24 hours for the stars to complete a revolution around the horizon*
*Of course, we now know that it is not the Sun and stars rotating around the Earth, but the Earth rotating and the rest of the universe remaining fairly static in the sky.
So the speed of Earth's rotation does not change between Summer and Winter.
The Earth acts like a gyroscope - the axis of Earth's rotation points in a fixed direction in space (when measured over the course of one year). This axis is currently tilted (http://en.wikipedia.org/wiki/Earth#Axial_tilt_and_seasons) at an angle of 23.4° to the ecliptic (the band across the sky which is followed by the Sun, Moon and stars).
- In mid-Winter, the poles faces away from the ecliptic, so they never see the Sun or planets. It is bitterly cold and depressing.
- In mid-Summer, the poles face towards the ecliptic, so they always see the Sun (and they still don't see the planets). It is still cold, and sleep deprivation is a real risk.
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Whose Summer? The southerners or the Northerners?
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Whose Summer? The southerners or the Northerners?
That made me think!
Ignoring daylight savings, locations in Africa (southern hemisphere) are in the same timezone as locations in Europe (northern hemisphere) - so they both spin at the same rate.
Africa (southern hemisphere) is in Summer while Europe (northern hemisphere) is in Winter, and vice-versa. So the Earth's spin must not change between Summer and Winter.
Logical, really!
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Whose Summer? The southerners or the Northerners?
That made me think!
Ignoring daylight savings, locations in Africa (southern hemisphere) are in the same timezone as locations in Europe (northern hemisphere) - so they both spin at the same rate.
Africa (southern hemisphere) is in Summer while Europe (northern hemisphere) is in Winter, and vice-versa. So the Earth's spin must not change between Summer and Winter.
Logical, really!
I considered a central observer rotating to a static light source at a constant rotational speed, it would be equal dark to equal light in time. I then imagined a sphere rotating at a constant speed, and a static light source, again day and night would be equal. I have considered raising the light and lowering the light, I am struggling to see how by the angle it works how it does without a rotational speed change?
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Think about the angle between the rotational axis and the light source.
If the axis is perfectly perpendicular to the line connecting the planet and light source, then yes, day and night would be exactly the same length for all points on the plant surface.
But if the axis points directly at the source of light, then it doesn't matter how quickly the planet spins, one side will always be dark and one side will always be illuminated.
If the axis is tilted anywhere in between, then some points will be illuminated longer than others, depending on their latitude...
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Think about the angle between the rotational axis and the light source.
If the axis is perfectly perpendicular to the line connecting the planet and light source, then yes, day and night would be exactly the same length for all points on the plant surface.
But if the axis points directly at the source of light, then it doesn't matter how quickly the planet spins, one side will always be dark and one side will always be illuminated.
If the axis is tilted anywhere in between, then some points will be illuminated longer than others, depending on their latitude...
Thank you, I understand the invert axis, I think I get it now, relative axis?
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This video shows how the length of daylight changes throughout the year.
- Daylight length does not really change at the equator: 12 hours of day+12 hours of night, all year round
- But if you look at polar regions, you will see that at some times of the year they are entirely in the dark, while the other pole is entirely in the light, all day long.
- At the equinox (twice per year), night and day are equal length, all over the world. We are approaching an equinox in a couple of weeks (about 22 March), so day and night are fairly equal now.
https: //www.youtube.com/watch?v=P-C9dykhTs8 [EDIT: remove space after colon, and paste into browser to watch]
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This video shows how the length of daylight changes throughout the year.
Wow , that's quite an interesting video, lol, I still can' t get me head around and visualise it in my head, I will just take your word on this one, it may come to me in time.
Thanks