Sky watch: How Earth's tilt causes seasons

At the end of next week, autumn officially arrives in the Earth's Northern Hemisphere.

when both hemispheres are equally illuminated -- marks the autumnal equinox in the Northern Hemisphere (spring equinox in the Southern Hemisphere). This year it's Friday at 5:05 a.m., and when the sun rises that morning, it will lie directly above our planet's equator.

For a few days, the sun will appear to rise due east and set due west. It is also around this time of year when our days and nights are of nearly equal length, and it is from this phenomenon that we get the term "equinox," Latin for "equal nights."

With no tilt, we would enjoy a more moderate and stable climate. Flora and fauna would become specialized, creatures of different sizes and types would intermingle more, and the ecology of our world would be quite different.

And if our planet's axis were tilted more? Seasonal changes would be more extreme; winds would blow more strongly, and storms would be more powerful and prevalent. Temperatures would vary sharply through the year with hot, tropical summers and blistering, arctic winters. Massive polar ice caps would form during winter and melt during summer, causing ocean levels to rise and drop by more than 100 feet every six months.