3.14159265359. If you've never seen this number, it's first few digits of the mathematical constant π (pi). And why is that interesting? Pi is the ratio of the circumference, the length around a circle, to its diameter, the length across a circle. When you do the math, you get a number that's slightly greater than 3. Since circles are fundamental to everything from building construction to quantum physics, pi sees a lot of use.

Pi also has a mysterious side. It's an irrational number. No matter the size of the circle, when you divide the length of the circumference by its diameter you'll always get the same number. And get this — you'll keep on dividing forever. There's no tidy "0" at the end. Pi keeps on going (as far as we know) with *no repeating pattern*. To date, pi lovers have divided it out to about 31.4 trillion decimal places. Given its uniqueness is it any wonder we celebrate Pi Day with pie every 3.14 (March 14)? Of course not! You can't deny that a nice, thick slice of blueberry pie would hit the spot right now.

Today also marks the return of the crescent moon to the evening sky. New moon phase occurred on March 13, when the moon lined up nearly in front of the sun. Only during a partial or total solar eclipse — when Earth, moon and sun lie along a straight line — are we able to see a new moon as a black silhouette blocking the sun. Now, a day and a half later, the moon has moved east and lies slightly to the left of the sun, its edge crisply illuminated by sunlight.

With binoculars you might even see the entire outline of the moon dimly lit by light reflected from Mother Earth. Moving east a little more than a fist a day as it orbits the Earth, the moon will mount higher and higher in the western sky in the coming nights. Fortunately, it won't be too bright in the coming week to spoil our next opportunity to see the northern lights.

I hope you had the chance to see either or both displays we had on Friday and Saturday nights, March 12-13. To keep you in the loop about potential auroras at any time, I write additional blogs beyond the three published each week by the News Tribune. You'll find them here on Blogger. Although the space weather forecast looks "quiet" for Sunday night, the aurora is expected to return between March 18-21. As always, keep an eye on the northern sky especially during the month of March, when northern lights are most frequent.

**"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.**