Earth's motion around the Sun, not as simple as I thought

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 on a path around the Sun that returns to its starting point so how do we know when a year starts or ends well one way called a Siberia 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 and when the Sun returns to its starting point a sight Tyrael 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 its dawn the annual motion is very noticeable the last stars seem 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 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 were to take 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 a maximum tilt towards or away from the Sun the length of the daylight is at a maximum or a minimum these days are called solstices and the Sun will be as the top level or bottom right of the analemma on the days when the earth's tilt is perfectly sideways to the Sun the day-night are equal in length these are the equinoxes and the Sun will be at this crossover 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 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 five thousand years ago the axis pointed to a star in the constellation Draco and twelve thousand years ago the brilliant star Vega was the pole star and because of the twenty six thousand year cycle Vega will be the pole star again and fourteen thousand years the precession of the equinoxes is caused primarily by gravitational forces of the Sun in the moon acting on the earth while the axial tilt is a primary cause of seasons on the earth the distance from the Sun which changes throughout the year because of the elliptical shape of the Earth's orbit contribute to small bit of temperature variations throughout the year as well when the axis is aligned so it points towards 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 entries in solar radiation but that same hemisphere will be in winner at aphelion and have a colder winner the other hemisphere will have a relatively warmer winter and cooler summer when the Earth's axis is aligned such that aphelion and perihelion occurred near the equinoxes the northern and southern hemisphere will have similar contrast in the seasons at present perihelion occurs during the southern hemispheres 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 in addition the gravitational effects of other planets caused the ellipse of our orbit to slowly spin around the Sun it takes about 112 thousand years for the ellipse to revolve once relative to fixed stars when considered together but two forms of precession ad and it takes about 21,000 years for the Solstice to go from aphelion to aphelion the dates of the perihelion and the aphelion advance each year on this cycle an average of one day for 58 years the eccentricity of the Earth's orbit is a measure of how round or how oval the orbits shape is over thousands of years the eccentricity of the Earth's orbit varies as a result of gravitational attractions among the planets primarily Jupiter and Saturn the orbital eccentricity cycles with a period of roughly 100,000 years as the eccentricity of the orbit involves the semi-major axis of the orbital ellipse remains unchanged so the length of the sidereal year remains unchanged as the earth travels in its orbit the duration of 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 are substantially longer in duration in addition to axial precession there's the axial tilt the angle the Earth's rotational axis makes with its orbital plane it is currently about twenty-three point four degrees and is declining this tilt varies from twenty-two point one degrees to twenty-four point five degrees it makes one complete tilt and back every forty-one thousand years 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 11800 ad the inclination of Earth's orbit drifts up and down relative to the present orbit with the cycle having a period of about 70,000 years and the orbit also moves relative to the orbits of other planets as well by calculating the plane of unchanged total angular momentum of the solar system we can define the orbital plane called the invariable plane it is approximately the orbital plane of Jupiter the inclination of the Earth's orbit has a 100 thousand year cycle relative to the invariable plane this 100 thousand year cycle closely matches the 100 thousand year pattern of ice ages a year on earth is directly determined by all the various orbital motions of the earth so if someone tells you how many years old they are you might ask them is that side aerial tropical or anomalous years

Channel: Aryan Navabi

Views: 8,210,872

Rating: 4.7425513 out of 5

Keywords: Earth's Motion, Earch and Sun, A year on earth, CassioPeia Project

Id: 82p-DYgGFjI

Length: 9min 28sec (568 seconds)

Published: Sat Jan 16 2010