The Westcave Solar Observatory

At the Westcave Visitor Center, one of the exhibit highlights is the Solar Observatory. This isn't about what's in the Sun, but how the Sun moves across the sky. This is one of the fundamental cycles of nature on our planet, and it influences much of the natural world that you see at the Preserve.

This observatory doesn't have a big dome, and it doesn't even have a big telescope.

If you visit us near the middle of the day, when the Sun is out, you'll see the Observatory in action. Way up on the ceiling we have a small hole. The Sun shines through the hole and makes a spot of sunlight on the floor. What's so special about that spot? Well, we can use it to watch the Sun move!

photo by R. Sprouse

Watch the spot closely, and you'll see it move across the floor. You're actually watching the Earth turn! In the middle of the day, at what we call "local noon", the spot will cross the meridian line down the center of the Visitor Center. This line points exactly north-south.

Now, that spot of light moves across the meridian line every day, but where it crosses depends on the time of year. In our summertime, the Sun gets high in the sky (which is why it's warm in the summer), and in our wintertime the Sun doesn't get as high. So where the spot crosses the meridian line changes day by day. The days are marked there, so it's like a calendar! (Don't worry if the spot doesn't cross the mark exactly for that date. There are 365 days marked on the floor, and as you know, sometimes we need to have 366 days in our year to make the calendar work!)

The reason the Sun is higher in the summer and lower in the winter is because the Earth is tilted as it goes around the Sun. In our summertime, centered on what we call the "summer solstice", the Sun is more overhead in our northern hemisphere than it is in our winter. You can see that when it is winter in the northern hemisphere, it is summer in the southern hemisphere!

So that's why the spot of sunlight tracks across the meridian in different places on different days.

Now, you might think that if that spot of light can tell you the date, it could also tell you the time, right? Well, that's a little complicated. Because the Earth's orbit around the Sun is not exactly a circle, and also because of that tilt that gives us seasons, the length of time it takes for the Sun to go all the way around the sky changes a little over the year.

In this figure, we're looking down at the floor of the Visitor Center, and we've shown where the spot of light is at exactly the same time of day over the course of the year. As we discussed above, in the winter the spot is off on one end, and in the summer it's at the other end, but it doesn't cross the meridian line at the same time every day. As you can see, the spot of light traces out a pattern on the floor (as the Sun does in the sky) called the "analemma". At Westcave, we've drawn the analemma on the floor at what we call "mean local noon", which at the longitude of Westcave is 12:32:34 CST. So when you see the spot of light cross the analemma curve at the listed date, you can set your watch to that time!

Now, there isn't anything that special about "mean local noon", except that averaged over the year, that's when the spot crosses the meridian line. If we decided to make marks at other clock times, we'd still get that figure-8 shape, but just shifted a bit.

At the Westcave Solar Observatory, we can chart the path of the Sun across the sky by watching the spot move across the floor. We can make the marks permanent because the motion of the Sun throughout the day and the year is very constant. Visitors a hundred years hence will see the spot of sunlight at the nearly the same place at the same time and date.

Enjoy your visit to Westcave Preserve, and bring the Sun!

830-825-3442 Fax: 830-825-3509

The Westcave Solar Observatory