Movement of Planet Earth

Hi Everybody,

Did you know that you would weigh 0.3% more if the Earth was standing still? Probably not, but it doesn’t matter because the Earth does spin. If you look at something on Earth, like a tree, or a house, it seems stationary. However, those objects, like everything else on Earth, are in constant motion. This is because the Earth, and everything on Earth, rotates every 23 hours, 56 minutes, and 4 seconds.

Hold On! We know that a day is 24 hours. How can approximately 4 minutes be missing from a day? Before we answer that, what is this day that has 23 hours, 56 minutes, and 4 seconds? This is called the sidereal day. Sidereal days have 24 sidereal hours which corresponds to a 15 degree movement of the stars per hour with respect to the Earth’s rotation. Basically, a sidereal day is a day with respect to the stars’ fixed positions. For example, if you take the star Deneb, and track its motion for a whole day, it would’ve made a complete 360 rotation after a sidereal day. This has been used since antiquity to determine time. However, why is it that we have 24 hour days, and not sidereal days?

As the Earth rotates, and sidereal days pass, it also moves along its orbit around the sun. If the Sun and Earth were aligned at noon, and a sidereal day passed, the Earth would be out of alignment with the sun. It would need an extra four minutes of rotation to realign it to the sun and reach noon. This is a solar day, and a solar day has 24 hours.

Sidereal vs Solar Day

Sidereal vs Solar Day

Another way to think about it is, as the Sun moves along the celestial sphere(the sky) from east to west, it trails by 4 RA minutes every day. The reason that we have these two standards of time is because, one clock runs 4 minutes fast. If September 21 is our start point, where midnight is 0 hr, 6 months later, on March 21, midnight is 12 hr, or noon. It creates an imbalance in time. It is simpler to have a 24 hr day where noon is noon every day of the year. It is easier to plan your daily events without taking into account the imbalance of time. It’s clear that the orbit of Earth has a profound impact on our day in many ways.

Earth orbits around the sun for one year or 365.25 days. During that orbit, it travels a total of 940 million km in space. Almost all orbits are not circular, but are ellipses. That is true for Earth as well. There are two points in any orbit called: Periapsis, and Apoapsis. (Each body has a different name for their orbital points, e.g. for Sun, Perihelion, and Aphelion). Earth has a perihelion of 147,098,290 km (0.98329134 AU) and an aphelion of 152,098,232 km (1.01671388 AU). On average, Earth has a semi-major axis of 149,598,261 km, which is 1.00000261 AU. This means its orbital eccentricity is 0.01671123, where an eccentricity of 0 is a perfect circle, anything between 0 and 1 represents an elliptical orbit, and anything 1 or greater is a parabolic orbit, or a hyperbolic orbit, respectively. While the orbit is slightly eccentric, this eccentricity doesn’t have much effect on the climate of Earth. It is seen in the fact that when the Earth is at aphelion (farthest point from the sun) in July, the Northern Hemisphere experiences summer, and when the Earth is at perihelion (the closest point to the sun), in January, the Northern Hemisphere experiences winter. The reverse is true for the southern hemisphere. The main reason why there is winter and summer is because of the tilt of the Earth.

During the year, there are four major events that occur: The Winter and Summer Solstices, and the Autumnal and Vernal Equinox. Imagine two lines on Earth, the equator, and the ecliptic. The ecliptic is 23.5 degrees tilted with respect to the equator. This simulation will help you imagine both lines. The equinox occurs when the sun reaches the point where the ecliptic and the equator intersect. At this time, both the Northern and Southern Hemispheres have an equally long day and night. The Solstices occur when the sun, on the ecliptic, is at the highest point away from the equator. At that point, it is angled towards one hemisphere and away from the other. For example, An observer in Canada, on June 21, will see the sun at its highest inclination, whereas an observer in Chile will see it at its lowest inclination. This means that the hemisphere that is tilted towards the sun will experience longer days and shorter nights, whereas the other hemisphere will experience shorter days and longer nights. Click here to see the seasons in action. This process repeats every 365.25 days, but not quite.

While an Earth year is 365.25 days, the definition of a year has been refined over the centuries. Earth has a sidereal year of 365.2564 days. This is the time it takes for the Earth to return to the same position with respect to the sun. For example, starting September 21, 1 sidereal year later, it will be in the same position. Our modern calendar year is 365 days. This is problematic because as these additional 1/4 days accumulate, it will add up and cause problems. Every four years you are off by day. After 720 years, you have 180 extra days, and that means January is summer in Northern Hemisphere, which is a problem if you are keeping time. Luckily, Julius Caesar was able to figure that out and is able to legislate the leap year. He has introduced the leap year, and as a result, we never drift more than a day. However, that is not enough.

Our calendar doesn’t drift more than a day, but it still doesn’t match the seasons. The calendar does take into account the length of the year, and the tilt of the Earth, but it doesn’t take into account the precession of the axial tilt. This precession causes the Earth’s tilt to rotate westward ever so slightly. This means that Polaris won’t be our north star forever. This precession completes its rotation every 26,000 years. While the effect is quite insignificant, it does affect our calendar, because year after year, the precession shortens the time between seasons. This results in the tropical year, which has 365.2422 days, which is slightly shorter than a sidereal year. As a result, to account for the axial precession, Pope Gregory XIII, in the 16th century, corrected for the difference between the sidereal year and tropical year by removing the leap years of centuries not divisible by 400. For example, years 1700, 1800, 1900 do not have leap years, but 1600, 2000 do have a leap year. This correction, and the tropical year is what the Gregorian calendar is based on, and this calendar allows our timekeeping to remain consistent for many years to come.

The path of Earth’s axial precession

What the precession looks like.

So what did we learn today? We learned a lot about the movement of Earth. We learned about what is a sidereal day, and what makes it different from a solar day. We learned about the orbit of Earth and what occurs during that orbit. We also learned about what makes our calendar the way it is today. I hope you all found this interesting.

Stay tuned for more blog posts…



Coursera Lecture – Week 1.4 – 1.7, 1.10


VIDEOS USED – NASAEarthObservatory