Download Practical Assignment SPU

We always define a year as ‘a trip from the earth around the sun’, but there are various ways
that can be measured. In this practical assignment, I will list some of the ways that people
use to measure years.
Tropical year
If you would take a stick, and put it vertically in the
earth, the sun will cast a shadow on it. If you
would take a picture of that shadow every day at
noon, you would see that they are slightly
different. This is because the sun’s position seen
from the earth changes due to the turning of the
earth. The noon suns move in a shape called an
annalemma (see picture). The highest point in the
annalemma is called the summer solstice, and the
lowest part is the winter solstice. the point where
they intersect is called the vernal or autumn
equinox. When the sun is at the equinox, the day and night are equally long. When the sun
has taken a full cycle, or the shadow from your stick is the same as it was when you started
taking photos of it, a tropical year(365.2422 days) has passed.
Sidereal year
A Sidereal year is the time it takes for the sun
to cycle through the twelve constellations of
the zodiac and return in the same position in
the sky compared to the other stars, viewed
from the earth. It is difficult to measure a
sidereal year, because when the sun is out,
you can’t see the stars, but if you look during
dawn, you can see the stars. A sidereal year
takes about 365.26 days, and the word
sidereal comes from the Latin word sidus,
meaning star.
Anomalistic year
As you can see on the picture, the earth’s orbit around
the sun is not perfectly round, but elliptical. The earth
doesn’t return to the starting point of the ellipse, but
advances a little bit. If you would calculate a year by the
time that the earth moves from apside – the extreme
point of the ellipse – to apside, you would get a period
that lasts about 365.259 days. We call this an
anomalistic year.
Draconic year
The draconic year, also called the ecliptic year, uses the
lunar node for measurement. The lunar nodes are the
points where the apparent line in which the moon traces
around the earth, the moon’s orbit, crosses the ecliptic.
When the sun crosses the exact same spot in the lunar
node than it did about a year(346.620 days) ago, a
draconic year has passed. Every half draconic year,
when the sun and the moon are near the lunar nodes, there is a chance of an eclipse.
Lunar year
A lunar year consists out of 12 complete cycles of the moon, called lunar months. When the
moon has been in all of its phases exactly 12 times, a lunar year has passed. A lot of
cultures, such as the Muslims and various Asian cultures used this as the main way to
measure a year, and still celebrate the lunar New Year.
Heliacal year
A heliacal year is measured by looking at the helical risings of the stars. When a star is just
visible above the horizon in the mere moments before dawn, the counting of a heliacal year
starts. In the days that follow, the star becomes visible a bit more and a bit longer. After a few
months, it isn’t visible anymore, because
the star has moved to far westwards. If
you keep observing the sky, you see that
eventually, the star will be seen again just
before dawn. A Heliacal year has passed.
A Sothic year is a variation on the Heliacal
year, using the star Sirius for measuring.
The Egyptians used this as a way of
measuring.
Galactic year
The galactic year is the time it takes for the sun to move around the milky way once. This
differs from 225 to 500 million Gregorian years (the years that are used in the western
calendar).
Conclusion
As you can see, there are lots of ways to define a year. They don’t differ a lot from each
other, but over the years, they certainly make a difference.
Sources:
http://astro.unl.edu/naap/motion3/sidereal_synodic.html
http://planetfacts.org/tropical-year/
http://www.youtube.com/watch?v=82p-DYgGFjI
http://en.wikipedia.org/wiki/Year
http://startswithabang.com/?p=1744