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Transcript

17- A Year On Earth

A few introductory words of explanation about this transcript.

This transcript includes the words sent to the narrator for inclusion in
the latest version of the associated video. Occasionally, the narrator
changes a few words on the fly in order to improve the flow. It is written
in a manner that suggests to the narrator where emphasis and pauses might
go, so it is not intended to be grammatically correct.

The Scene numbers are left in this transcript although they are not
necessarily observable by watching the video.

There will also be occasional passages in blue that are NOT in the video
but that might be useful corollary information. There may be occasional
figures that suggest what might be on the screen at that time.

110 Intro

A year on earth is measured by one complete trip around the sun. Seems
simple enough… but there is a problem. The earth doesn’t travel in a
path around the sun that returns it to its starting point. So how do we
know when the year starts or ends?

115 Sidereal Year

Well one way – called a sidereal year -- measures our orbit against the
distant stars. As viewed from the earth, our orbit causes the sun to
appear to move through the constellations of the zodiac on a path called
the ecliptic (which is just the plane of earth’s orbit). And when the
sun returns to its starting point, a sidereal year has passed.

This motion is difficult to observe directly because the stars cannot be
seen when the Sun is in the sky. However, if you look at the sky before
each dawn, the annual motion is very noticeable. The last stars seen to
rise are not always the same, and within a week or two an upward shift can
be noted. As an example, in July in the Northern Hemisphere, Orion cannot
be seen in the dawn sky, but in August it becomes easily visible.

Measuring a year this way, gives a period that is 365 days, 6 hours, 9
minutes and 10 seconds long.

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120 Tropical Year

Another possibility is to measure the year against the passing of the
seasons.

Because of the tilt of the earth on its axis, the position of the sun in
the sky changes from day to day throughout the year. If we took a picture
of the sun at noon regularly throughout the year we would see the sun
moving on this path – called an analemma.

On the days in its orbit when the earth is at maximum tilt towards or away
from the sun, the length of daylight is at a maximum or minimum. These
days are called solstices, and the sun will be at the top left or bottom
right of the analemma.

On the days when the earth’s tilt is perfectly sideways to the sun, the
day and night are equal in length. These are the equinoxes, and the sun
will be at a latitude-dependent position in the analemma.

When the sun goes from one vernal equinox to the next, a tropical year has
passed. Measuring this way, gives a year that is 365 days, 5 hours, 48
minutes and 46 seconds long.

125 LuniSolar Precession

The length of a year on earth is affected by several gradual and cyclical
changes in its orbit and its tilt.

First, there is the precession of the earth’s axis. Over a period of
about 26,000 years, the earth’s axis traces out a circle in the sky. One
result of this is that the North Star changes over time. Right now the
Earth’s axis points towards Polaris. 5000 years ago the axis pointed to
a star (Thuban) in the constellation Draco.

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And 12,000 years ago the brilliant star Vega was the pole star – and
because of the 26,000-year cycle, Vega will be the pole star again in
14,000 years.

The precession of the equinoxes is caused primarily by gravitational
forces of the sun and the moon acting on the earth. While the axial tilt
is the primary cause of seasons on earth, the distance from the sun
–which changes throughout the year because of the elliptical shape of
the earth’s orbit – contributes a small bit to temperature variations
throughout the year as well.

When the axis is aligned so it points toward the Sun during perihelion,
one hemisphere will have a greater difference between the seasons while
the other hemisphere will have milder seasons. The hemisphere which is in
summer at perihelion will receive much of the corresponding increase in
solar radiation, but that same hemisphere will be in winter at aphelion
and have a colder winter. The other hemisphere will have a relatively
warmer winter and cooler summer.

When the Earth's axis is aligned such that aphelion and perihelion occur
near the equinoxes, the Northern and Southern Hemispheres will have
similar contrasts in the seasons.

At present, perihelion occurs during the Southern Hemisphere's summer, and
aphelion is reached during the southern winter. So the Southern Hemisphere
seasons are somewhat more extreme than the Northern Hemisphere seasons,
when other factors are equal. Perihelion presently occurs around January
3, while aphelion is around July 4

The International Astronomical Union recommended that the dominant
component be renamed the precession of the equator and the minor component
be renamed precession of the ecliptic, but their combination is still
named general precession. Anomalistic precession refers to the rotational
movement through space of the apsides of a celestial body's orbit.

There is also NUTATION

The plane of the Moon's orbit about the Earth rotates with respect to the
ecliptic with a period of 18.6 years. This causes the Earth to nod, a
motion that is superimposed on the LuniSolar precession.

130 Anomalistic or Planetary Precession (perihelion or apsidal precession)
In addition, the gravitational effects of the other planets cause the
ellipse of our orbit to slowly spin around the sun.

It takes about 112,000 years for the ellipse to revolve once, relative to
the fixed stars.

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When considered together, the two forms of precession add, and it takes
about 21,000 years for the solstice to go from aphelion to aphelion.

The dates of perihelion and of aphelion advance each year on this cycle,
an average of 1 day per 58 years.

140 Orbital Eccentricity

The eccentricity of the Earth’s orbit is a measure of how round or how
oval the orbit shape is. Over thousands of years, the eccentricity of the
Earth's orbit varies (from nearly 0.0034 to almost 0.058) as a result of
gravitational attractions among the planets – primarily Jupiter and
Saturn. It is currently close to its mean value.

The orbital eccentricity cycles with a period of roughly 100,000 years. As
the eccentricity of the orbit evolves, the semi-major axis of the orbital
ellipse remains unchanged, so the length of a sidereal year remains
unchanged.

Currently the difference in distance from the sun at closest and farthest
approach results in a 6.8% change in incoming solar radiation. Perihelion
presently occurs around January 3, while aphelion is around July 4. When
the orbit is at its most elliptical, the amount of solar radiation at
perihelion is about 23% greater than at aphelion. This difference is
roughly 4 times the value of the eccentricity.

As the earth travels in its orbit, the duration of the seasons depends on
the eccentricity of the orbit. When the orbital eccentricity is extreme,
the seasons that occur on the far side of the orbit (aphelion) are
substantially longer in duration.

Today, northern hemisphere fall and winter occur at closest approach
(perihelion), when the earth is moving at its maximum velocity. As a
result, in the northern hemisphere, fall and winter are slightly shorter
than spring and summer. In 2006, summer was almost 5 days longer than
winter -- and spring is almost 3 days longer than fall. Axial precession,
already mentioned, slowly changes the place in the Earth's orbit where the
solstices and equinoxes occur.

Over the next 10,000 years, northern hemisphere winters will become
gradually longer and summers will become shorter.

145 Nutation

The plane of the Moon's orbit about the Earth rotates with respect to the
ecliptic with a period of 18.6 years. This causes the Earth to nod, a
motion that is superimposed on the LuniSolar precession.

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145 Axial Tilt (Obliquity)

In addition to axial precession, there is the axial tilt – the angle the
Earth’s rotational axis makes with its orbital plane. It is currently
about 23.4 degrees and is declining. The tilt varies from 22.1 degrees to
24.5 degrees. It makes one complete tilt and back every 41,000 years.
Since the tilt towards and away from the sun is the primary cause of the
seasons, more tilt means more solar radiation gets to the poles and less
tilt means less radiation gets to the poles. So This change in tilt is
directly related to ice ages on Earth. The last maximum tilt occurred in
8700 BC and the next minimum tilt will happen in11,800 AD.

150 Orbital Inclination

The inclination of Earth's orbit drifts up and down relative to its
present orbit with a cycle having a period of about 70,000 years. And the
orbit moves relative to the orbits of the other planets as well.

By calculating the plane of the unchanging total angular momentum of the
solar system, we can define an orbital plane called the invariable plane.
It is approximately the orbital plane of Jupiter. The inclination of the
Earth's orbit has a 100,000 year cycle relative to the invariable plane.
This 100,000-year cycle closely matches the 100,000-year pattern of ice
ages.

It has been proposed that there is a disk of dust and other debris is in
the invariable plane, and this affects the Earth's climate through several
possible means. The Earth presently moves through this plane around
January 9 and July 9. The present ecliptic plane is inclined to the
invariable ecliptic plane by about 1.5°

160 Conclusion

A year on earth is directly determined by all the various orbital motions
of the earth, so … if somebody tells you how many years old they are –
you might ask them “Is that in sidereal, tropical or anomalous years?”