I recently found this video on the NOAA GOES archive website: http://www.goes.noaa.gov/ and overlaid some geometry to define the alignment of Earth and Sun. It nicely shows the interplay of night and day on an equinox.
The view is from a GOES satellite located in geostationary orbit around 35,786 km (22,236 mi) above sea level. GOES stands for “Geostationary Operational Environmental Satellite” and this particular satellite is “parked” in an equatorial orbit with a view of the Americas. At any given time there are 15-20 GOES satellites operational, each parked with a view covering different overlapping swaths of the planet. Their main purpose is long-term monitoring of weather, climate, pollution, and other global variables.
This particular video captures the Vernal Equinox on March 20 of this year. While keeping the Americas in view, it shows the sweep of daylight across the western hemisphere from sunrise through sunset on the U.S. East Coast.
Of note is the alignment of the terminator (border between day and night) with Earth’s polar axis — the axis around which it rotates. On an equinox, every point on the planet, save for near the North and South Poles, experiences 12 hours of daylight and 12 hours of darkness. Hence the term equinox — literally “equal night” from the Latin. You can see how the terminator remains exactly aligned with the lines of longitude that run from pole to pole. On an equinox, Earth’s polar axis is aligned perpendicular to the light source.
Near the poles things get a little more complicated. The Sun is visible for 24 hours, but it grazes along the horizon with the upper half of the Sun visible and the lower part below the horizon. Check out this video from the South Pole: https://vimeo.com/136274402
There are many ways to visualize an equinox, and I’ve done most of them here in Sky Lights. You can run a search for “equinox” in the Sky Lights archives and find them all, but my two favorites are:
Equinox Sunset and
Balancing an Egg on the Spring Equinox
By comparison, on a solstice, the polar axis is tilted by 23.5° with respect to the light source. You can see an animation of that in my July 18, 2016 post about the Midnight Sun.
Next Week in Sky Lights ⇒ Astrophotography Then and Now