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Discussion Starter #1 (Edited)
This FAQ attempts to shed some light on just what this Solstice thing is all about. Well, really, I attempt to explain what the Solstices are, and the mechanism by which they occur.

This set of posts is not for the faint of heart, they're LONG! So don't say I didn't warn you.

Because of limitations in the length of posts, I've had to break this up into multiple postings.

Enjoy!
 

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Discussion Starter #2 (Edited)
For the truly impatient [I don't care how it works, just show me the table!]

The times for the Solstices and Equinoxes are shown at: US Naval Observatory: Earth's Seasons...1992-2020. Now, when your reading the chart, note that the times are displayed in Coordinated Universal Time pretty much the same as [GMT] or the times in Greenwich England. So for the Eastern US Time Zone, subtract 4 hours (when Daylight Saving Time is in effect [Summer Solstice and Autumnal Equinox] and subtract 5 hours when Eastern Standard Time is in effect [Winter Solstice and Vernal Equinox]. For those out in the west, the Daylight Saving Time offset is subtracting 7 hours and for Standard Time, it's subtracting 8 hours.

So for example, the Summer Solstice for 2005 occurs 2005-06-21 06:46:00Z or
  • 2005-06-21 02:46:00-04 EDT
  • 2005-06-21 01:46:00-05 CDT
  • 2005-06-21 00:46:00-06 MDT
  • 2005-06-20 23:46:00-07 PDT
So, the sunrise on 2005-06-21 is the Summer Solstice sunrise this year.

See the post below on the Significance of the Solstices for a discussion of the reason that the sunrise itself is the celebrated event, rather than the precise moment in the astronomical sense.
 

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Discussion Starter #3
Brief Introduction

In brief, the earth's axis is tipped at 23.5 degrees relative to the plane of its orbit. As the Earth orbits the Sun, in Summer Months in the Northern Hemisphere, that tilt is oriented more toward the Sun, and in Winter months in the Northern Hemisphere, the North Pole is pointed somewhat away from the Sun. So, as the Earth rotates, the apparent position of the rising and setting points of the Sun track in a pendulum like manner from a more Southerly point in the winter to a more Northerly point in the Summer. [I'm speaking about the Northern Hemisphere here, so the positions and seasons are reversed for our Australian, African, and South American colleagues.]
 

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Discussion Starter #4
Origin of the Word Solstice

The term Solstice is actually made up of two root words.
http://www.religioustolerance.org/winter_solstice.htm said:
"Solstice" is derived from two Latin words: "sol" meaning sun, and "sistere," to cause to stand still.
So, you can now guess, that the Solstices occur when that pendulum like motion of the rising and setting points of the Sun (as they track back and forth across the horizon) is at its most extreme Northern and Southern points along the horizon. As the date of each of the Solstices approaches the Sun rises near those points, with each day leading up to the Solstices seeing the rising and setting points creep more slowly Northward or Southward toward their extreme points. Finally, on the day of the Solstice, the motion of the rising and setting points along the horizon appears to stop and reverse course. Notice I said STOP. That's the "stice" part of the name.

Now, let's dig a bit deeper and see why this is happening....
 

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Discussion Starter #5
Where Does the Earth's Axis Point?

Now, to understand this motion of the Sun's rising and setting points, you might think that the Earth's axis is pointing in different directions throughout the year. Well, no, it's not....

The axis does wobble in a 23,000 year precession like a top [see NASA.gov Precession] with the axis swinging through a fairly regular circle. At this time, the Northern end of the Earth's axis of rotation is pointed at the star at the end of the handle of the little dipper also known as the constellation Ursa Minor (the little bear [it's a special bear with a really long tail, but it's legend, so who knows...]) [For a bit more background, see: [NASA.gov Finding the Pole Star ]. In this era, we call that star Polaris because the pole points at it. To find Polaris, check out the guide at: US Office of Naval Research: Finding Polaris. Be sure to click on the grey "Play >" button, and the red ">" arrows in the text box popups to keep things moving along.

Will Polaris always be the pole star? No, the pole will swing around to Alrai in 2000 years, and to Alderamin in about 5500 years, Vega in ... Well, you get the idea, and here's a link which shows what's going on: US Office of Naval Research: Changes of the North Star

What that means is that if you looked up at Polaris all night long, it wouldn't move, and the other stars (and planets) would appear to move around it, taking exactly one solar day to make a full circle [though it would be a bit difficult to see the stars once the sun rose, but you would be able to see that the Sun too was swinging in a great circle across the sky and seemed to make a circle that was centered on the now washed out Polaris].

Why did we talk about Polaris first? Well, we need to establish that the Earth's axis points in the same direction at all times when measured on a human life span scale. To be precise, in your life time, you would be hard pressed to see the change in the position of Polaris relative to the center of the circles all the other stars are making. So, you can take it as red that, the Earth's axis really doesn't change the direction it's pointed while the Earth orbits the Sun. That means that the 23.5 degree inclination of the axis to the plane of the Earth's orbit around the Sun is constant. To see the effect of that inclination during Winter in the Northern Hemisphere, look at: US Office of Naval Research: Motion of the Earth - Rotation.
 

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Discussion Starter #6 (Edited)
Visualizing the Tilt

Now, what does all of this have to do with the Summer and Winter Solstices? Well, that tilt (inclination of 23.5 degrees relative to the plane of the orbit of the Earth around the Sun) means that as it orbits, the Earth's axis always leans in the same direction.

To help you visualize this, imagine having someone stand in the center of an empty room. You stand near the center of one wall and lean your head toward the opposite wall. Notice, your head is now closer than your feet to the person in the center of the room. Now, walk around the person in the center, but keep leaning toward that same wall you leaned toward before you started walking, as you walk toward the side wall. Now, stop when you get to the center of that other wall. At this point, you'll be leaning toward that original wall, but not toward the person in the center any more, just toward the wall you started leaning toward when we started this whole charade. Looking at the person in the center, your head is no longer leaning toward them, and your feet and your head are about the same distance from the person in the center of the room. Now, continue your trip around the room toward the wall you've been leaning toward this whole time, and stop when you get to the center of that wall. If you now turn and face the person in the center, you'll notice that to lean toward that same wall you've been leaning toward through all of this, you have to lean your head away from the person in the center--your feet are now closer to that person than your head. OK, that's enough of the leaning, but I think you got the point--you were able to lean toward one wall the whole time, and you had three different orientations relative to the person in the center. First, you were leaning toward them, then you were leaning to the side, then you were leaning away from them. That's what the Earth's axis does as the Earth travels around the Sun.

To see a cool explanation, check this link: US Office of Naval Research: Motion of the Earth - Seasons

So, in the Northern Hemisphere, the Summer Solstice occurs when the Earth is in that part of its trip around the sun where the Earth's North Pole is leaned toward the Sun. This is similar to when we were standing in the center of the wall before we started our leaning trip around the room above and we were leaning toward the opposite wall, and consequently toward the person in the center.

Likewise, in the Northern Hemisphere, the Winter Solstice occurs when the Earth's orbit takes it all the way around to the other side of the Sun (with the Earth's axis still pointing in the same direction [at Polaris]) and the North Pole is tilted away from the Sun. This is analogous to where we stopped our leaning trip around the room leaning toward that same wall, but this time, standing directly in front of that wall, we wound up leaning our head away from the person in the center.
 

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Discussion Starter #7 (Edited)
Why the Tilt Changes Where the Sun Sets

Now, seeing why the point where the Sun rises and sets moves North and South along the horizon is a bit more tricky. To do that, we have to look look closely at the little animation of the earth as it rotates on its axis (remember, the axis always points in the same direction when measured on the scale of a human life time).

So, run the little animation at: US Office of Naval Research: Motion of the Earth - Seasons by clicking on the green "Play" in the upper left side of the top row of the image. What we need to see here is that as the Earth's axis is always leaning the same way as it goes around the Sun. While it's a bit faint, you should be able to make out the dashed white line of the Earth's axis as it circles the Sun, and it is always pointed to the upper right.

In the coming steps, we will click on the little red words under each season that say "Close Up".

From this point, it gets a bit more difficult to imagine what's going on.

Let's start with the close up of Autumnal Equinox (click on the "Close Up" under the Autumnal Equinox on the image.)

Now, for the following discussion, we're going to have to work with the sunsets because the animations show the sunset side of the Earth.

Notice that the shadow line falls straight up and down, and is aligned with the white dots representing the Earth's axis. The axis is actually leaned into the picture away from you, but it's still pointing the same direction (at Polaris). The shadow line falling straight up and down lined up with the Earth's axis means that as the Earth turns you into the shadow, the Sun will be exactly due West. So, the Sun will set exactly due West of you as you watch it sink below the horizon. To see this, take a ruler and lay it horizontally across your screen, and watch for your continent to swing around into view. Now, as your particular location crosses into the shadow, notice where the left edge of the ruler is lined up with. It will be due west of your location. So if you're in Florida, it points more or less just below the the bottom edge of Southern California.

Close the window by clicking on "Close Window" and let's go to the next diagram, the "Close Up" of the Winter Solstice.

Now it gets tricky to see, but for those in the Northern Hemisphere, you can see that the shadow line is still vertical in the picture, but the Earth's Axis is leaned off to the right. Remember, it's still pointing in the same direction, at Polaris, but now were at a different place on the Earth's orbit, and the Earth's North Pole is leaned away from the Sun. So, if you watch as your continent swings by, you can see that at sunset, the sun would be somewhat south of West when it set. Lay a ruler horizontally across the screen parallel to the bottom of the screen and watch for your continent to swing by the shadow line. When it swings by, you will note that the line the ruler makes from your location will point South West across that continent. For our Florida folks, that means that the ruler will line up more with the southern tip of Baja California. So, the sun is setting in the South West, and you'd have to be facing a bit South of West to watch it disappear below the horizon.

Close the window by clicking on "Close Window" and let's go to the next diagram, the "Close Up" of Vernal Equinox.

This time, the Earth's axis is once again lined up with the shadow line in the picture. Don't forget that it's still pointing in the same direction, namely at Polaris, but our view point has been swung around. What's a bit harder to make out is that the Earth's axis is actually leaning out toward you a bit, and we can see more of the Northern Hemisphere in this shot. Once again, we lay our ruler horizontally across the picture, and we see that the Sun has returned to setting due West as we watch our continent swing by into the shadow.

Close the window by clicking on "Close Window" and let's go to the next diagram, the "Close Up" of the Summer Solstice.

Note that the Earth's Axis as shown by the dashed white line is in a different position on the drawing. But remember, it's still pointed in the same direction, this time, however our view is from the back side of the main orbit diagram which shows the earth circling around the sun. We're actually looking out of the screen, instead of into it. (Yeah, I know it's a bit tough visualize, but if you don't get what I meant by looking out of the screen instead of into it, don't worry, you only need to remember that that white dashed line is still pointed at Polaris)

Here, we see the North Pole leaned toward the Sun. That means that when we lay our ruler horizontally across the picture, we see the opposite of what we saw with the Winter Solstice. So, lay the ruler across the screen horizontally, and watch for your continent to come swinging into view. Sometimes, it's slightly easier to lay the ruler above your spot on the continent where it crosses the shadow line, and let's say you're in Florida as some of our folks are, notice that the ruler is pointing more towards Oregon than it is toward Southern California. This means that as you watched the Sun set in Florida, the direction you would have to face to watch that sunset would be somewhat North of due West, roughly in the direction of Oregon.

There's a little more to it as well, and the following links may help you visualize what's going on. One point that we didn't cover is the elevation of the sun as it sweeps across the sky. In the summer months, it sweeps higher across the sky in the Northern Hemisphere and in the Winter, it swings through the sky somewhat lower.

For seeing the path, and the rising and setting points as if you viewed them from the ground, check out: Dr. Pamela Gore's page, The Motion of Objects in the Sky and the sun path diagram that she borrowed from NASA at: NASA.gov From Stargazers to Starships, The Angle of the Sun's Rays.
 

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Discussion Starter #8 (Edited)
Significance of the Solstices

Our ancestors knew about the Solstices (no not the cars) and created observatories to help them track the motions. The most famous is Stonehenge, but there are many others. See archaeoastronomy.com Seasons as a starting point. (Keep in mind that on some of their drawings and animations, they are displaying times and dates in GMT)

These ancient observatories were used to identify the days of the Solstices and Equinoxes by when the Sun rose and set over certain points on the Horizon. These were marked by alignments often with large stones. Stonehenge even uses alignments looking between the pillars.

In ancient times, the Solstices were an extremely important part of life. Today, they mark the starts of our seasons just as they were associated with the seasons our ancestors used for determining when to plant and when to harvest.

For our forefathers, it was the Solstice sunrise and sunsets that were the significant events. Today, we can more accurately measure the event, and can calculate the occurrence with far greater precision. That's why you will see not only a day listed in the tables linked in the first section of this thread, but the hour and minute as well. However, it is still the magic of the Sunrise on the Summer and Winter Solstices that we celebrate today, following a tradition predating written history.
 

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Post your response to this thread here.

For further information, the "plural" of Solstice is "Solstices", as in:

"I like to sip a cup of tea to celebrate both the summer and winter solstices - a hot one in winter, and iced in the summer."

Or:

"That dealer has five Solstices on his lot, yet he won't come down to invoice."
 
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