When in the far north several years ago, I found out an interesting thing about the moon.

I knew that in northern latitudes in the summer it would be difficult to see the moon because the sun is out nearly 24 hours a day. The full moon is much dimmer than the sun at midday, about 465,000 times dimmer. I'm told that if the night sky were completely filled with full moons, it would still be six times as dark as when our one sun is outÐan interesting statement about the capacity of human sight, which can function quite well outside by the light of one full moon.

I also know that despite this difference in brightness, if you look for it you can see the moon when it is out during the day near its full phase. The only reason you can't see both the sun and the full moon at the same time is because the full moon rises just as the sun sets. I figured that in the Arctic summer the sun doesn't set, so both the sun and the full moon would be out at the same time. I was very surprised then when I couldn't see the moon. I stood there in the Arctic with the sun out at midnight, and the full moon refused to show itself. In fact, as I began to look for it over the next several days, I found that the moon was never out. I was baffled.

Actually, it turns out that same effect is at work here. The moon appears lower in the sky in the summer and higher in the sky in the winter, the reverse of what the sun does. In the Arctic, it is so low that I couldn't find it. It didn't ever rise above the high mountains to the south of where we were. I had heard vivid descriptions of moonlight in the winter there. This seasonal change in the moon's height happens because of some interesting things that the earth and moon do in relation to the sun as they go about their orbit.

To explain this effect, consider that the earth has a tilt of about 23 degrees from the plane of its orbit around the sun. This tilt is responsible for the seasonsÐwhen in its orbit the earth's north pole is tilted away from the sun, there is less area that the sun can illuminate in the northern hemisphere, and the effect is that our days are shorter and the air is cooler. In summer, or in the southern hemisphere, this effect is reversed. Last Saturday marked our summer solstice, where in the northern hemisphere the earth is tilted most directly toward the sun.

To make matters even more interesting, the moon has an inclination in its orbit to the earth. Imagine the moon is on the edge of a plate with the earth in the center, and that plate is tilted from the earth's equator about 5 degrees. Without this inclination, the moon would always eclipse the sun when it orbited around the earth, and the earth would eclipse the moon. Because of this inclination, we have just a few eclipses, between two and five solar eclipses when the moon passes in front of the sun, and up to three lunar eclipses when the earth passes in front of the moon. These can be partial or total eclipses. Meanwhile, this inclination of the moon's orbit shifts with the earth's orbit of the sun, so that the moon is ten degrees lower in the sky in the summer than it is in the winterÐjust enough to make it drop below the mountains where I was visiting in the Arctic summer.

The full moon near the summer solstice is called the "Strawberry Moon". It seems to mark a time in our lives when we are apt to be enjoying some of our more memorable moments. Whether it is the warm long days, the moon, or some other reason, it is a great time of year.