Keeping Time

In a previous lesson, we learned that the sun moves relative to the stars, completing one full cycle around the entire celestial sphere each year. As a reminder, you can re-watch the Apparent Motion of the Sun video.
In addition to moving eastward relative to the stars, the sun also moves higher and lower in the sky over the course of the year. Read 4.2 The Seasons to understand the role of Earth’s axial tilt in creating the seasonal cycle, explaining how the sun’s position affects day length and heating throughout the year.
I’m sure you’ve learned about the seasons in previous science classes, but you’ve probably never tried to use terms like “declination” and “meridional altitude” to make sense of the sun’s changing position in the sky. Watch the Seasons and Ecliptic Simulator video to see how all of these ideas and terminology connect to one another. You can also use the simulator yourself.
It’s one thing to see the changing angle of sunlight in a simulation, but here’s a real time-lapse video clip from space, showing the changing illumination of the earth over an entire year.
Ooh, here’s another cool time-lapse video of the sun’s motion. Check out this Solar Declination Video, which dramatically illustrates how the changing declination of the sun (and therefore its altitude) impacts the seasons. The images at left were captured at noon each day over the course of an entire year. Notice how the change of seasons aligns with the changing height of the sun in the sky (and the length of the noontime shadow of the building).
Watch the video Motion of the Sun which uses a comprehensive simulation to tie together all we have learned about the sun’s motion. If you want, you can explore the Motions of the Sun Simulator for yourself.
Wow - that’s a lot to take in and put together. Once again, I’d like to show you what this actually looks like in the sky. So, watch the video Real Sky Solar Motion, which illustrates the changing declination of the sun in the sky.
Explore 4.3 Keeping Time for insights into how solar motion affects global timekeeping.
Learn more about the historical efforts to align day, month, and year lengths by reading 4.4 The Calendar.