Autumn officially began in the Northern Hemisphere last week, and days are getting shorter. This month, explore what causes day and night.

Mystery Class begins January 28, 2013. Watch for monthly "Countdown to Mystery Class" messages to help you and your students get ready for the challenge.

Day and Night
Image: A Clevenger

Report your October 1st photoperiod:

• Look up your data for October 1.
• Report data (select "daylight").
• View data on photoperiod map.

Observe, Wonder, and Predict
Observe images of the Earth from space. See how sunlight strikes the Earth over the course of a day. Use the slideshow and activities to explore the 24-hour cycle of changes.

• Slideshow: What Makes Day and Night?

• How to Participate: Step-by-step Guide
• Practice Packet #1: Latitude Clues
• Practice Packet #2: Longitude Clues