[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:07.03,0:00:10.04,Default,,0000,0000,0000,,Suppose you placed a camera\Nat a fixed position,
Dialogue: 0,0:00:10.04,0:00:11.59,Default,,0000,0000,0000,,took a picture of the sky
Dialogue: 0,0:00:11.59,0:00:15.04,Default,,0000,0000,0000,,at the same time everyday \Nfor an entire year
Dialogue: 0,0:00:15.04,0:00:18.61,Default,,0000,0000,0000,,and overlayed all of the photos \Non top of each other.
Dialogue: 0,0:00:18.61,0:00:21.63,Default,,0000,0000,0000,,What would the sun look like\Nin that combined image?
Dialogue: 0,0:00:21.63,0:00:23.03,Default,,0000,0000,0000,,A stationary dot?
Dialogue: 0,0:00:23.03,0:00:24.71,Default,,0000,0000,0000,,A circular path?
Dialogue: 0,0:00:24.71,0:00:25.70,Default,,0000,0000,0000,,Neither.
Dialogue: 0,0:00:25.70,0:00:28.53,Default,,0000,0000,0000,,Oddly enough, it makes this \Nfigure eight pattern,
Dialogue: 0,0:00:28.53,0:00:30.56,Default,,0000,0000,0000,,known as the Sun's analemma,
Dialogue: 0,0:00:30.56,0:00:32.38,Default,,0000,0000,0000,,but why?
Dialogue: 0,0:00:32.38,0:00:35.01,Default,,0000,0000,0000,,The Earth's movement \Ncreates a few cycles.
Dialogue: 0,0:00:35.01,0:00:39.32,Default,,0000,0000,0000,,First of all, it rotates on its axis\Nabout once every 24 hours,
Dialogue: 0,0:00:39.32,0:00:41.98,Default,,0000,0000,0000,,producing sunrises and sunsets.
Dialogue: 0,0:00:41.98,0:00:44.74,Default,,0000,0000,0000,,At the same time,\Nit's making a much slower cycle,
Dialogue: 0,0:00:44.74,0:00:49.82,Default,,0000,0000,0000,,orbiting around the sun\Napproximately every 365 days.
Dialogue: 0,0:00:49.82,0:00:51.07,Default,,0000,0000,0000,,But there's a twist.
Dialogue: 0,0:00:51.07,0:00:53.16,Default,,0000,0000,0000,,Relative to the plane of its orbit,
Dialogue: 0,0:00:53.16,0:00:57.40,Default,,0000,0000,0000,,the Earth doesn't spin \Nwith the North Pole pointing straight up.
Dialogue: 0,0:00:57.40,0:01:02.86,Default,,0000,0000,0000,,Instead, its axis has a constant tilt\Nof 23.4 degrees.
Dialogue: 0,0:01:02.86,0:01:06.84,Default,,0000,0000,0000,,This is known as the Earth's axial tilt,\Nor obliquity.
Dialogue: 0,0:01:06.84,0:01:09.86,Default,,0000,0000,0000,,A 23-degree tilt may not seem important,
Dialogue: 0,0:01:09.86,0:01:13.82,Default,,0000,0000,0000,,but it's the main reason that\Nwe experience different seasons.
Dialogue: 0,0:01:13.82,0:01:16.70,Default,,0000,0000,0000,,Because the axis remains tilted\Nin the same direction
Dialogue: 0,0:01:16.70,0:01:19.06,Default,,0000,0000,0000,,while the Earth makes its annual orbit,
Dialogue: 0,0:01:19.06,0:01:20.78,Default,,0000,0000,0000,,there are long periods each year
Dialogue: 0,0:01:20.78,0:01:24.60,Default,,0000,0000,0000,,when the northern half of the planet\Nremains tilted toward the Sun
Dialogue: 0,0:01:24.60,0:01:27.20,Default,,0000,0000,0000,,while the southern half is tilted away
Dialogue: 0,0:01:27.20,0:01:28.35,Default,,0000,0000,0000,,and vice versa,
Dialogue: 0,0:01:28.35,0:01:31.95,Default,,0000,0000,0000,,what we experience as summer and winter.
Dialogue: 0,0:01:31.95,0:01:33.100,Default,,0000,0000,0000,,During summer in a given hemisphere,
Dialogue: 0,0:01:33.100,0:01:38.71,Default,,0000,0000,0000,,the Sun appears higher in the sky,\Nmaking the days longer and warmer.
Dialogue: 0,0:01:38.71,0:01:40.91,Default,,0000,0000,0000,,Once a year, the Sun's declination,
Dialogue: 0,0:01:40.91,0:01:42.68,Default,,0000,0000,0000,,the angle between the equator
Dialogue: 0,0:01:42.68,0:01:46.69,Default,,0000,0000,0000,,and the position on the Earth\Nwhere the Sun appears directly overhead
Dialogue: 0,0:01:46.69,0:01:48.46,Default,,0000,0000,0000,,reaches its maximum.
Dialogue: 0,0:01:48.46,0:01:53.38,Default,,0000,0000,0000,,This day is known as the summer solstice,\Nthe longest day of the year,
Dialogue: 0,0:01:53.38,0:01:57.13,Default,,0000,0000,0000,,and the one day where the Sun \Nappears highest in the sky.
Dialogue: 0,0:01:57.13,0:01:58.86,Default,,0000,0000,0000,,So the Earth's axial tilt
Dialogue: 0,0:01:58.86,0:02:02.79,Default,,0000,0000,0000,,partially explains why the Sun \Nchanges positions in the sky
Dialogue: 0,0:02:02.79,0:02:04.22,Default,,0000,0000,0000,,and the analemma's length
Dialogue: 0,0:02:04.22,0:02:09.25,Default,,0000,0000,0000,,represents the full 46.8 degrees\Nof the sun's declination
Dialogue: 0,0:02:09.25,0:02:10.66,Default,,0000,0000,0000,,throughout the year.
Dialogue: 0,0:02:10.66,0:02:13.90,Default,,0000,0000,0000,,But why is it a figure eight\Nand not just a straight line?
Dialogue: 0,0:02:13.90,0:02:17.12,Default,,0000,0000,0000,,This is due to another feature\Nof the Earth's revolution,
Dialogue: 0,0:02:17.12,0:02:19.46,Default,,0000,0000,0000,,its orbital eccentricity.
Dialogue: 0,0:02:19.46,0:02:22.19,Default,,0000,0000,0000,,The Earth's orbit around the Sun\Nis an ellipse,
Dialogue: 0,0:02:22.19,0:02:26.04,Default,,0000,0000,0000,,with its distance to the Sun\Nchanging at various points.
Dialogue: 0,0:02:26.04,0:02:28.78,Default,,0000,0000,0000,,The corresponding change\Nin gravitational force
Dialogue: 0,0:02:28.78,0:02:32.31,Default,,0000,0000,0000,,causes the Earth to move \Nfastest in January
Dialogue: 0,0:02:32.31,0:02:34.81,Default,,0000,0000,0000,,when it reaches \Nits closest point to the Sun,
Dialogue: 0,0:02:34.81,0:02:36.35,Default,,0000,0000,0000,,the perihelion,
Dialogue: 0,0:02:36.35,0:02:40.23,Default,,0000,0000,0000,,and the slowest in July\Nwhen it reaches its farthest point,
Dialogue: 0,0:02:40.23,0:02:42.62,Default,,0000,0000,0000,,the aphelion.
Dialogue: 0,0:02:42.62,0:02:45.15,Default,,0000,0000,0000,,The Earth's eccentricity \Nmeans that solar noon,
Dialogue: 0,0:02:45.15,0:02:47.79,Default,,0000,0000,0000,,the time when the Sun \Nis highest in the sky,
Dialogue: 0,0:02:47.79,0:02:50.97,Default,,0000,0000,0000,,doesn't always occur \Nat the same point in the day.
Dialogue: 0,0:02:50.97,0:02:54.41,Default,,0000,0000,0000,,So a sundial may be as much\Nas sixteen minutes ahead
Dialogue: 0,0:02:54.41,0:02:58.19,Default,,0000,0000,0000,,or fourteen minutes behind\Na regular clock.
Dialogue: 0,0:02:58.19,0:03:04.20,Default,,0000,0000,0000,,In fact, clock time and Sun time\Nonly match four times a year.
Dialogue: 0,0:03:04.20,0:03:08.62,Default,,0000,0000,0000,,The analemma's width represents\Nthe extent of this deviation.
Dialogue: 0,0:03:08.62,0:03:11.74,Default,,0000,0000,0000,,So how did people know \Nthe correct time years ago?
Dialogue: 0,0:03:11.74,0:03:13.64,Default,,0000,0000,0000,,For most of human history,
Dialogue: 0,0:03:13.64,0:03:16.33,Default,,0000,0000,0000,,going by the Sun's position\Nwas close enough.
Dialogue: 0,0:03:16.33,0:03:17.84,Default,,0000,0000,0000,,But during the modern era,
Dialogue: 0,0:03:17.84,0:03:22.08,Default,,0000,0000,0000,,the difference between sundials\Nand mechanical clocks became important.
Dialogue: 0,0:03:22.08,0:03:25.42,Default,,0000,0000,0000,,The equation of time, \Nintroduced by Ptolemy
Dialogue: 0,0:03:25.42,0:03:28.44,Default,,0000,0000,0000,,and later refined based \Non the work of Johannes Kepler,
Dialogue: 0,0:03:28.44,0:03:34.42,Default,,0000,0000,0000,,converts between apparent solar time and\Nthe mean time we've all come to rely on.
Dialogue: 0,0:03:34.42,0:03:37.85,Default,,0000,0000,0000,,Globes even used to have \Nthe analemma printed on them
Dialogue: 0,0:03:37.85,0:03:40.00,Default,,0000,0000,0000,,to allow people to determine \Nthe difference
Dialogue: 0,0:03:40.00,0:03:44.56,Default,,0000,0000,0000,,between clock time and solar time\Nbased on the day of the year.
Dialogue: 0,0:03:44.56,0:03:48.68,Default,,0000,0000,0000,,Just how the analemma appears\Ndepends upon where you are.
Dialogue: 0,0:03:48.68,0:03:51.76,Default,,0000,0000,0000,,It will be tilted at an angle\Ndepending on your latitude
Dialogue: 0,0:03:51.76,0:03:54.23,Default,,0000,0000,0000,,or inverted if you're in \Nthe southern hemisphere.
Dialogue: 0,0:03:54.23,0:03:55.78,Default,,0000,0000,0000,,And if you're on another planet,
Dialogue: 0,0:03:55.78,0:03:58.38,Default,,0000,0000,0000,,you might find something \Ncompletely different.
Dialogue: 0,0:03:58.38,0:04:01.84,Default,,0000,0000,0000,,Depending on that planet's \Norbital eccentricity and axial tilt,
Dialogue: 0,0:04:01.84,0:04:04.35,Default,,0000,0000,0000,,the analemma might appear as a tear drop,
Dialogue: 0,0:04:04.35,0:04:05.26,Default,,0000,0000,0000,,oval,
Dialogue: 0,0:04:05.26,0:04:07.14,Default,,0000,0000,0000,,or even a straight line.