The Sun’s surprising movement across the sky - Gordon Williamson
-
0:07 - 0:10Suppose you placed a camera
at a fixed position, -
0:10 - 0:12took a picture of the sky
-
0:12 - 0:15at the same time everyday
for an entire year -
0:15 - 0:19and overlayed all of the photos
on top of each other. -
0:19 - 0:22What would the sun look like
in that combined image? -
0:22 - 0:23A stationary dot?
-
0:23 - 0:25A circular path?
-
0:25 - 0:26Neither.
-
0:26 - 0:29Oddly enough, it makes this
figure eight pattern, -
0:29 - 0:31known as the Sun's analemma,
-
0:31 - 0:32but why?
-
0:32 - 0:35The Earth's movement
creates a few cycles. -
0:35 - 0:39First of all, it rotates on its axis
about once every 24 hours, -
0:39 - 0:42producing sunrises and sunsets.
-
0:42 - 0:45At the same time,
it's making a much slower cycle, -
0:45 - 0:50orbiting around the sun
approximately every 365 days. -
0:50 - 0:51But there's a twist.
-
0:51 - 0:53Relative to the plane of its orbit,
-
0:53 - 0:57the Earth doesn't spin
with the North Pole pointing straight up. -
0:57 - 1:03Instead, its axis has a constant tilt
of 23.4 degrees. -
1:03 - 1:07This is known as the Earth's axial tilt,
or obliquity. -
1:07 - 1:10A 23-degree tilt may not seem important,
-
1:10 - 1:14but it's the main reason that
we experience different seasons. -
1:14 - 1:17Because the axis remains tilted
in the same direction -
1:17 - 1:19while the Earth makes its annual orbit,
-
1:19 - 1:21there are long periods each year
-
1:21 - 1:25when the northern half of the planet
remains tilted toward the Sun -
1:25 - 1:27while the southern half is tilted away
-
1:27 - 1:28and vice versa,
-
1:28 - 1:32what we experience as summer and winter.
-
1:32 - 1:34During summer in a given hemisphere,
-
1:34 - 1:39the Sun appears higher in the sky,
making the days longer and warmer. -
1:39 - 1:41Once a year, the Sun's declination,
-
1:41 - 1:43the angle between the equator
-
1:43 - 1:47and the position on the Earth
where the Sun appears directly overhead -
1:47 - 1:48reaches its maximum.
-
1:48 - 1:53This day is known as the summer solstice,
the longest day of the year, -
1:53 - 1:57and the one day where the Sun
appears highest in the sky. -
1:57 - 1:59So the Earth's axial tilt
-
1:59 - 2:03partially explains why the Sun
changes positions in the sky -
2:03 - 2:04and the analemma's length
-
2:04 - 2:09represents the full 46.8 degrees
of the sun's declination -
2:09 - 2:11throughout the year.
-
2:11 - 2:14But why is it a figure eight
and not just a straight line? -
2:14 - 2:17This is due to another feature
of the Earth's revolution, -
2:17 - 2:19its orbital eccentricity.
-
2:19 - 2:22The Earth's orbit around the Sun
is an ellipse, -
2:22 - 2:26with its distance to the Sun
changing at various points. -
2:26 - 2:29The corresponding change
in gravitational force -
2:29 - 2:32causes the Earth to move
fastest in January -
2:32 - 2:35when it reaches
its closest point to the Sun, -
2:35 - 2:36the perihelion,
-
2:36 - 2:40and the slowest in July
when it reaches its farthest point, -
2:40 - 2:43the aphelion.
-
2:43 - 2:45The Earth's eccentricity
means that solar noon, -
2:45 - 2:48the time when the Sun
is highest in the sky, -
2:48 - 2:51doesn't always occur
at the same point in the day. -
2:51 - 2:54So a sundial may be as much
as sixteen minutes ahead -
2:54 - 2:58or fourteen minutes behind
a regular clock. -
2:58 - 3:04In fact, clock time and Sun time
only match four times a year. -
3:04 - 3:09The analemma's width represents
the extent of this deviation. -
3:09 - 3:12So how did people know
the correct time years ago? -
3:12 - 3:14For most of human history,
-
3:14 - 3:16going by the Sun's position
was close enough. -
3:16 - 3:18But during the modern era,
-
3:18 - 3:22the difference between sundials
and mechanical clocks became important. -
3:22 - 3:25The equation of time,
introduced by Ptolemy -
3:25 - 3:28and later refined based
on the work of Johannes Kepler, -
3:28 - 3:34converts between apparent solar time and
the mean time we've all come to rely on. -
3:34 - 3:38Globes even used to have
the analemma printed on them -
3:38 - 3:40to allow people to determine
the difference -
3:40 - 3:45between clock time and solar time
based on the day of the year. -
3:45 - 3:49Just how the analemma appears
depends upon where you are. -
3:49 - 3:52It will be tilted at an angle
depending on your latitude -
3:52 - 3:54or inverted if you're in
the southern hemisphere. -
3:54 - 3:56And if you're on another planet,
-
3:56 - 3:58you might find something
completely different. -
3:58 - 4:02Depending on that planet's
orbital eccentricity and axial tilt, -
4:02 - 4:04the analemma might appear as a tear drop,
-
4:04 - 4:05oval,
-
4:05 - 4:07or even a straight line.
- Title:
- The Sun’s surprising movement across the sky - Gordon Williamson
- Description:
-
View full lesson: http://ed.ted.com/lessons/the-sun-s-surprising-movement-across-the-sky-gordon-williamson
Suppose you placed a camera at a fixed position, took a picture of the sky at the same time every day for an entire year, and overlaid all of the photos on top of each other. What would the sun look like in that combined image? A stationary dot? A circular path? Neither. Oddly enough, it makes a ‘figure 8’ pattern, known as the Sun’s analemma. Gordon Williamson explains why.
Lesson by Gordon Williamson, animation by TED-Ed.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TED-Ed
- Duration:
- 04:23
Jessica Ruby approved English subtitles for The Sun’s surprising movement across the sky - Gordon Williamson | ||
Jessica Ruby edited English subtitles for The Sun’s surprising movement across the sky - Gordon Williamson | ||
Jessica Ruby accepted English subtitles for The Sun’s surprising movement across the sky - Gordon Williamson | ||
Jessica Ruby edited English subtitles for The Sun’s surprising movement across the sky - Gordon Williamson | ||
Jessica Ruby edited English subtitles for The Sun’s surprising movement across the sky - Gordon Williamson | ||
Jennifer Cody edited English subtitles for The Sun’s surprising movement across the sky - Gordon Williamson | ||
Jennifer Cody edited English subtitles for The Sun’s surprising movement across the sky - Gordon Williamson |