# @worldscifest on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/BKqvJAsB5Of
- **Gondola URL:** https://gondola.cc/posts/2461192-worldscifest-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/635d0a01ec.jpg
- **Posted:** 2016-09-22T18:10:13.000+00:00
- **Account Owner:** WorldScienceFestival (@WorldSciFest) — https://gondola.cc/WorldSciFest

## Caption

Happy #equinox and happy #fall to all!
#Repost @nasa_eo ・・・
First Day of Fall 2016

One of the most frequently misunderstood concepts in science is the reason for #Earth’s seasons. As we experience the September #equinox today we thought we’d offer a space-based view of what’s going on.

Around 6 a.m. local time each day, the Sun, Earth, and any geosynchronous satellite form a right #angle, affording a nadir (straight down) view of the terminator, the edge between the shadows of nightfall and the #sunlight of dusk and dawn. The shape of this line between night and #day varies with the seasons, which means different lengths of days and differing amounts of warming #sunshine. (The line is actually a curve because the Earth is round, but satellite #images only show it in two-dimensions.) The Spinning Enhanced Visible and Infrared Imager (#SEVIRI) on #EUMETSAT's Meteosat-9 captured these views of Earth from geosynchronous orbit. The images show how sunlight fell on the Earth between September 19, 2010 and September 19, 2011. Each image was taken at 6:12 a.m. local time.

Of course, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and its nearest #star. The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the December #solstice and toward the Sun at the June solstice, spreading more and less light on each #hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the #light is spread evenly.

The equinox and changing of the #seasons occurs on September 22, 2016 at 10:21 a.m. ET (2:21 p.m. UTC). Equinox means "#equal #night" in Latin, capturing the idea that daytime and nighttime are equal lengths everywhere on the #planet. That is true of the Sun's presence above the horizon, though it does not account for twilight, when the #Sun's rays extend from beyond the #horizon to illuminate our gas-filled #atmosphere.

http://go.nasa.gov/21BYv8e

@nasa #earthnow #nasaearth #science

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## Tags

light, night, images, earth, angle, sunlight, star, day, seviri, atmosphere, seasons, horizon, nasaearth, equinox, earthnow, repost, solstice, planet, equal, eumetsat, sun, hemisphere, fall, science, sunshine

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