The following instructions assume that you have Celestia160-ED (being released in February 2010), or Celestia151-ED, plus the latest version of Lua Edu Tools 1.2. If not, you can still do this with the regular version of Celestia (160 or 151), with a slight modification of instructions. Details are below.
Feel free to copy/paste or print these instructions as you see fit.
Directions for Displaying the Advance of the Seasons in Celestia:
1. You first need to position yourself above Earth, at a distance that suits the view you wish. To do that, launch Celestia or Celestia-ED by clicking on the program icon. Press the  key which selects Earth, then press the [G] key to go there. You will fly to Earth and be positioned in orbit above it. Celestia will lock on and follow Earth as it orbits the sun.
2. Press the [ Y ] key to synch orbit Earth. This will keep the same face/view toward you as you watch the seasons pass.
3. Use your [mouse wheel] (or the [home ]or [end ]keys) to zoom in or out from that view so the distance is what you want. I suggest you turn off the clouds, so you can see the ground and continents well. To turn off clouds, just press the [ i ] key.
4. Then, turn on a grid on Earth by [right-clicking] on Earth, select "Alternate Surfaces", and choose "Grid Overlay". A grid of Earth with Equator, both Tropics and both Circles drawn and labeled, will appear. Note: This grid is included with Celestia-ED. If you don't have Celestia-ED, you can download and install the grid by visiting the Earth addons page at the Celestia motherlode website, or you can use the built-in planet grid that comes with Celestia (see below).
5. Next, you will need to decide your latitude and longitude viewpoint. I suggest you use the orientation controls [right-click-drag], [left-click-drag] and the [ left arrow ] and [ right arrow ] keys to tilt your view of Earth downward, so the Arctic Circle and Pole are visible. Then drag left or right so you are above the longitude you want. If the time of day is not right for the view, just speed up time with the [ L ] key. When the time of day is what you want, return to normal time with the [ \ ] key.
6. You can mark the North Pole if you wish by right-clicking on Earth again, selecting "Reference Vectors", and choosing "Show Body Axis". Three arrows will appear coming out of Earth. The blue one comes out of the North Pole. You can also turn on a "Terminator" line through the same menu, which draws a line where the hazy terminator is located. The Vectors menu also has its own latitude and longitude grid (Planetographic Grid), if you prefer that.
7 Set the date that you wish to begin the discussion of the seasons (e.g. - I suggest a solstice or equinox). Dates can be set by clicking on the "Set Time" button at the top right of the white on-screen Lua Edu Tools toolkit. A small menu will appear. Click on the month position in the date menu, and an arrow bar will appear below it, allowing you to change the month. Ditto for the day or year. If you don't have Lua Edu Tools running, you can adjust the date through the Time menu at the top of Celestia.
Once all the above is adjusted, you can then show the class how the dark side and terminator changes on Earth, as you advance a month or a day at a time. It works really well.
8. You can technically speed up time really fast and just watch the terminator shadow move across the pole, down and up, reaching the Arctic Circle line, then reversing course. However, the problem is that Earth rotates, so you will also see the planet spinning like a top as the shadows change. It makes things harder to see. I recommend advancing the date a month at a time.
9. When you are done the lesson, you must [right-click] on Earth, choose "Alternate Surfaces" again, and select "Normal". You should also turn off the reference vectors, and restore the clouds by pressing the [ i ] key again.
10. With a bit of training in script writing, it is possible to write a script that will change the dates for you a month or a week at a time, at the press of a key.
Hope this is useful. Enjoy!!