Here comes the sun…

A post by Anne JeffersonThe Earth’s axis has a 23.44o obliquity or tilt to it. As the Earth revolves around the Sun over the course of a year, the axial tilt means that different parts of the Earth’s surface receive direct sunlight at different times of the year. And it’s this receipt of varying intensities of solar radiation that drives temperature differences, and hence seasonality.
Today is a solstice, illustrated by the image on the far right below. Today is the day of the year when the Northern Hemisphere is tilted farthest away from the sun and the Southern Hemisphere is tilted most towards the sun. For those of us in the Northern Hemisphere, it’s our shortest day of the year and the sun never gets very high in the sky, even at noon. In fact, the word solstice has a Latin origin in the word solstitium, where “sol” means sun and “stitium” means stoppage. and for several days around the solstice this noontime elevation appears to be the same – hence the stoppage. Today, the noontime sun appears directly overhead along the Tropic of Capricorn, 23.44o S.
Figure 1. Earth at the solstices and equinoxes, as seen from the north. Source: Wikimedia.
The precise moment of the solstice occurs at 17:47 UTC (12:47 pm Eastern Standard Time). We’ll have another solstice (image on far left) on 21 June 2010 at 11:28 UTC ( 7:28 am Eastern Daylight Time). Over the course of the Earth’s trip around the Sun there will be two moments when everybody is getting their fair share of sunlight – the equinoxes. In 2010, they’ll occur on 20 March 2010 at 17:32 UTC and 23 September 3:09 UTC (22 September 11:09 pm EDT).
Earth’s tilt also varies over geologic time. It has a ~41-thousand year cycle, and right now we’re at about the middle of the range in variation of axial tilt. As tilt increases, seasonal contrasts over much of the world increase, but it is decreased axial tilt is tied with the onset of continental glaciation. That’s because at high latitudes, when tilt is low, summers are even cooler, and more snow persists through the summer. That surviving snow forms the nucleus of glacial ice caps. We’re currently on the decreasing limb of the obliquity cycle, but based on past occurrence of continental glaciations, the onset of another one is going to require not just less obliquity, but also the right eccentricty and precession in the Earth’s orbital parameters and controlling greenhouse gas emissions.
Figure 2: Last seen at Clastic Detritus in 2007, original created by Slumbering Lungfish.

Categories: by Anne, planets
Tags: , , ,

Comments (7)

  1. Lab Lemming says:

    Merry Christmas everyone. Do I win the nerd of the week award for downloading that image and rotating it in GIMP to check the tilt angle?

  2. Lab Lemming,
    Yes. Yes, you do.

  3. IanW says:

    “Today is a solstice, illustrated by the image on the far right below”
    That image on the right looks like Earth’s north pole is tilted towards the sun. Or are my eyes just weird?

  4. IanW: At the December solstice, the southern hemisphere is tilted towards the sun and the northern hemisphere is tilted away from it. One easy way to tell is that the Arctic Circle is in full darkness while Antarctica is in full light.

  5. James says:

    I think it depends whether the sun is on the left or right hand side. I’m supposing it’s on the right in this image.

  6. Phil Warnell says:

    A nice post that has one wonder as to what would the general populous’ reaction be to the prospect of a quick yet natural onset of an impending ice age. This is not a thought born of someone being a climate change denier, yet to simply wonder what the consensus would be in regards to the inevitable consequence of such changes; that if we had the power to mitigate them would we be so inclined . My thoughts being in as the repercussions being imilar or even worse in respect to all life forms, not simply our own, it would be in contradiction for those that press for action currently not to want to.

  7. It was very dissapointing. It could be a while before any meaningful treaties are signed. Another 10 years or so of rising temps might be enough to convince people. But in the mean time I am really concerned now that temperatures may rise significantly above the 2c over the next few decades. We can always hope that some technoligical breakthroughs will save us, but it’s not something we can count on.