Fall, also known as autumn, arrives today at 10:44:18 a.m. This is one of two days during the year when we have equal amounts of day and night (the other is in March at the start of spring).

We have four seasons because the Earth is tilted 24 degrees on its axis. Imagine a top standing straight up on a floor. Now, tilt the top slightly to the left or the right. The Earth is tilted in a similar fashion while circling the sun.

Because of that tilt, there are times during the year when the Earth is getting a lot of sunlight during the day, when it's getting very little sunlight during the day and when it's getting an equal amount of sunlight during the day.

Those times define our seasons which, astronomically speaking, start on the solstices and equinoxes.

The summer solstice marks the position in the Earth's orbit when its axis is most directly tilted toward the sun. This is the 24-hour period of the greatest amount of sunlight during the year, more commonly known as the longest day of the year.

The winter solstice marks the position in the Earth's orbit when its axis is most directly tilted away the sun. This is the 24-hour period of the least amount of sunlight during the year, more commonly known as the shortest day of the year.

The spring and fall equinoxes mark the two positions in the Earth's orbit when the sun appears vertically above the equator. This is the 24-hour period of equal amounts of day and night.

The Internet is a great resource for astronomical information on the seasons:

What Causes The Seasons

www.crh.noaa.gov/fsd/astro/season.php

A great explanation from the National Weather Service office in Sioux Falls, S.D.

Wolfram Research: Season

http://scienceworld.wolfram.com/astronomy/Season.html

Short but technical explanation for the seasons.

Earth's Seasons

http://aa.usno.navy.mil/data/docs/EarthSeasons.php

A list of the astronomical starts of winter, spring, summer and fall through 2020. Times given are in 24-hour Universal Time, meaning you'll need to subtract five hours to get Central Daylight Time and subtract six hours to get Central Standard Time.

InfoPlease: Equinox, Seasons

www.infoplease.com/spot/equinox1.html

www.infoplease.com/ipa/A0002064.html

A good astronomical overview of the seasons.

Bad Astronomy: Seasons

www.badastronomy.com/bad/misc/badseasons.html

Site owner Phil Plait doesn't think the seasons are bad; what needs to be changed is when they start.

Reason For The Seasons

http://online.chabotcollege.edu/shildreth/astronomy/06seasons.html

A neat college worksheet on astronomy and the seasons. A brief quiz also features detailed answers and explanations. A good way to learn about the seasons.

Equinox

http://en.wikipedia.org/wiki/Equinox

The information is detailed and, surprisingly, quite accurate.

Birthday fun online

I recently celebrated a birthday, which got me to thinking - and that's always a dangerous thing (just ask those who know me). I'm starting to think "age" and "old" are relative terms, especially since I'm, well, getting older. If I'm in a room with people at or under the age of 20, I'd be considered old, very old. But if I'm in a room with people at or older than 70, I'd be considered a babe in the woods.

So I created a site where you can compare how "old" or "young" you are based on life expectancies in the past and life expectancies in other countries. For example, if you're 40 years old, you might think of yourself today as being in your prime. But in the U.S. in 1900, you'd be quite old - life expectancy was only 48 to 51 at that time.

You can check out the age comparison page at www.darnay.com/birthday2. And you can also check out my "older than/younger than" birthday script at www.darnay.com/birthday.

Try Chrome

if you haven't yet downloaded and tried the new free "Chrome" web browser from Google, I'd encourage you to do so. It's fast, it's quite stable and it reflects how people use web browsers today. Your browser home page in Chrome is a dynamic starting point that always shows the top nine sites you regularly visit. In this way, most of the places you go to on a regular basis are just an image click away. With Chrome, web pages load faster than in the other browsers. Try it out and compare it to the browsers you're currently using - I think you'll be impressed.

Go to www.google.com/ chrome/ to download the browser. One note: There's no Mac version yet of the Chrome browser.

(Keith Darnay is the webmaster and designer for bismarcktribune.com. His Web site, featuring this column going back to 1995, is at www.darnay.com.iec.)