It’s hard to see a pattern in most of the constellations. Their stars are too faint or too spread out, or the pattern is just too obscure. Perhaps the most prominent exception is Scorpius. It takes little imagination to see the curving body of a scorpion in its stars.

The scorpion skitters low across the south on summer nights. Its brightest star is Antares. The scorpion’s body and tail curl to the lower left. The head is to the upper right. It’s marked by a line of three stars. They’re about the same brightness, and they’re fairly evenly spaced.

From top to bottom, the stars are Beta, Delta, and Pi Scorpii. Delta is a bit brighter than the others.

All three stars are extraordinary. Each of them actually consists of more than one star. All of the member stars are quite young – no more than a few percent the age of the Sun. And most of them are big and heavy, with some of them fated to end their lives as supernovas – titanic explosions that will outshine billions of normal stars.

Delta Scorpii consists of two stars. At least one of them will become a supernova. Pi Scorpii is a triple system. It also features at least one future supernova.

Beta is the busiest of the systems – at least six stars, all orbiting each other in a complex gravitational ballet. Two of those stars are likely to become supernovas – briefly highlighting the head of the scorpion.

Script by Damond Benningfield

Our clocks tick off a steady 24 hours per day. But if a sundial could record the time with the same accuracy, it would show that the length of the day changes. The difference is called the equation of time.

Clocks measure the length of a day averaged over a full year – the Sun’s average motion across the sky. Sundials show the Sun’s true motion. Over the course of a year, the length of a solar day – the period from one local noon to the next – varies by almost a minute. And that adds up. In early February, a solar day lasts about 14 minutes less than 24 hours. In early November, it lasts about 16 and a half minutes more than 24 hours.

The change has a couple of causes. Earth’s orbit is lopsided, so our planet travels at different speeds. When we’re closest to the Sun, we move faster than average; when we’re farthest, we move slower. But the rate at which Earth spins on its axis remains the same. The difference in those two motions causes the Sun to move a little faster or slower across the sky, changing the length of a solar day.

And Earth’s axis is tilted, so the poles take turns dipping toward the Sun. Today is the June solstice, so the north pole is tilting sunward. The change in the Sun’s position as a result of that tilt adds to the complexity.

The solar day is exactly 24 hours long around June 13th. So now, the equation of time is almost zero – a close match between the sundial and the clock.

Script by Damond Benningfield

Summer arrives here in the United States in the wee hours of tomorrow morning – the moment of the June solstice. At the solstice, the Sun stands farthest north for the entire year. For people at about 23-and-a-half degrees north latitude, our star will pass directly overhead at local noon.

That line of latitude is known as the Tropic of Cancer. It was named a couple of thousand years ago. At the time, the Sun appeared against the constellation Cancer at the solstice. Today, though, the Sun’s almost directly astride the border between Taurus and Gemini. It’s on the Taurus side at the exact moment of the solstice, but it slides into Gemini a few hours later.

The change in address is the result of a slow “wobble” in Earth’s axis. As it wobbles, the Sun shifts position against the background of stars. It takes our planet about 26,000 years to complete a single wobble, so that’s how long it takes the Sun to move all the way across the zodiac. So the Sun will return to Cancer in about 24,000 years.

Earth’s axis also nods up and down a little over an even longer period – about 41 thousand years. That causes a shift in the latitude of the Tropic of Cancer. Right now, it’s moving southward at about 50 feet per year – changing the circle where the Sun stands overhead on the summer solstice.

We’ll have more about the summer solstice tomorrow.

Script by Damond Benningfield