Summer Solstice & Earths axis tilt...

[Ella]

When exactly does Earths axis tilt... is it today or tomorrow...

[jflowers]

This year, the summer solstice is 11.39pm on the 20th (for me anyway at 52 degrees) I guess its similar for the rest of the UK?

[Ella]

Thanks I read it was today after 11pm but not sure... so does it slowly tilt at this time & how long does it take to tilt?

[JamesF]

It's always tilted It doesn't change. The fact that the Earth orbits the Sun does mean that the position of the sun relative to a given point is always changing though.

James

[JamesF]

Well, actually, it does change a little bit, but not in a manner that's relevant here...

James

[Ella]

Thanks James... I thought it shifts/tilts twice a year... so is the position of the sun...

[JamesF]

It's hard to explain this without pictures, but if you imagine the Sun at the centre of a dinner plate and the Earth travelling around the edge, and we'll say it's midwinter when the Earth is as far to your left as it can go and midsummer when it's as far to your right as it can go. The Earth is always tilted, and if we take midwinter as a starting point, the north pole will be tilted away from the sun whilst the south pole is tilted towards it, by 23.5 degrees from the vertical, so the north pole points "out" to your left and the south pole points "in" to your right. As the year progresses the Earth will move towards you around the edge of the dinner plate, but the direction of tilt doesn't move with it. At the spring equinox when the Earth is closest to you, the tilt is still north pole to the left, south pole to the right. Come midsummer it's the same story. North left, south right.

The effect of the tilt is that in midwinter the north pole is pointing "away" from the sun, but in midsummer it's pointing "towards" it. The effect of having circled halfway around the sun is that the position of the north pole has changed relative to the sun, but it hasn't actually changed at all relative to the position it was in six months previously.

Does that make any sense?

James

[JamesF]

Wikipedia have a handy animated gif here:

http://en.wikipedia.org/wiki/File:Earth_tilt_animation.gif

James

[Ella]

Thanks again James... this makes alot of sense... always thought it tilted not by so much but is actually less than thought...

[Ad Astra]

Earth's axis tilt (and the seasons it causes) are easy to explain. You can do it with a martini and an olive.

Put the martini glass in the center - this is the Sun. Now take your olive with the toothpick in it (the toothpick is Earth's axis) and point it straight up. This would model the Earth with no axis tilt. Move the olive around the glass while keeping the toothpick axis straight up all the time. In this no-tilt scenario, our olive-Earth gets exactly 12 hours of daylight and darkness every day without fail, no matter where the olive is in its orbit.

Now tilt the toothpick axis about 30 degrees - instead of pointing the toothpick up at 12 o' clock, tilt it to 11 o' clock. Again, move it slowly around the glass -- without every allowing the direction of the toothpick to change. You will easily see that for part of the orbit, the northerly end of olive-Earth's axis is pointing somewhat toward the Sun; this is summer with its long days and short nights. For part of the year, the northerly end of the toothpick axis points away from the Sun, this is northern winter. The point where the axis points most directly at (or away from) the Sun are the solstices. The longest and shortest day of the year respectively. You will also notice that when we have our summer solstice, our mates in Australia (the whole southern hemisphere, actually) are having their summer. This is why seasons are reversed below the equator. You will also note that when the olive's position is 90-degrees away from the solstice point, the toothpick does not point either toward or away from the glass - these are the equinoxes when we have exactly 12 hour days and nights.

The last step of our tabletop astronomy experiment is easy. Drink the martini, enjoy the olive, set up your scope for the evening and contemplate the wonders of the Universe from the comfort of the garden!

Cheers! (literally!)

Dan

[Ella]

Earth's axis tilt (and the seasons it causes) are easy to explain. You can do it with a martini and an olive.

Put the martini glass in the center - this is the Sun. Now take your olive with the toothpick in it (the toothpick is Earth's axis) and point it straight up. This would model the Earth with no axis tilt. Move the olive around the glass while keeping the toothpick axis straight up all the time. In this no-tilt scenario, our olive-Earth gets exactly 12 hours of daylight and darkness every day without fail, no matter where the olive is in its orbit.

Now tilt the toothpick axis about 30 degrees - instead of pointing the toothpick up at 12 o' clock, tilt it to 11 o' clock. Again, move it slowly around the glass -- without every allowing the direction of the toothpick to change. You will easily see that for part of the orbit, the northerly end of olive-Earth's axis is pointing somewhat toward the Sun; this is summer with its long days and short nights. For part of the year, the northerly end of the toothpick axis points away from the Sun, this is northern winter. The point where the axis points most directly at (or away from) the Sun are the solstices. The longest and shortest day of the year respectively. You will also notice that when we have our summer solstice, our mates in Australia (the whole southern hemisphere, actually) are having their summer. This is why seasons are reversed below the equator. You will also note that when the olive's position is 90-degrees away from the solstice point, the toothpick does not point either toward or away from the glass - these are the equinoxes when we have exactly 12 hour days and nights.

The last step of our tabletop astronomy experiment is easy. Drink the martini, enjoy the olive, set up your scope for the evening and contemplate the wonders of the Universe from the comfort of the garden!

Cheers! (literally!)

Dan

A LOL education to our solstice... must try this with a pint of cider nut & stick... now I need to visit a bar...