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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