Do earth revolves around sun with variable speed?

Question

We know that in summer we are close to sun and in winter we are far from sun. We also know that the gravitational force is inversely proportional to distance. It means that sun's gravitational force on earth during winter (Aphelion) is less as compared to summer ( Perihelion). According to Newton's gravitational law, centrifugal and centripetal forces should be equal in order to have body in circular motion. Centrifugal force depends on Velocity of object. So does it mean that Velocity of earth DECREASES in winter ( Aphelion) as compared to Summer ( perihelion) in order to balance the centrifugal and centripetal forces?????????

Answer 1

We were at aphelion on July 3rd (Northern Hemisphere Summer and Southern Hemisphere Winter) and perihelion on January 4th (Northern Hemisphere Winter and Southern Hemisphere Summer), So there is no correlation between seasons and distances from the sun, Seasons are caused by the tilt of the earth's axis.

However, you have grasped the point that rotational speed in the earth's orbit around the sun does vary according to our position in that orbit.

Kepler's second law states a line joining a planet and its star sweeps out equal areas during equal intervals of time. This is also known as the law of equal areas.

Suppose a planet takes one day to travel from points A to B. During this time, an imaginary line, from the Sun to the planet, will sweep out a roughly triangular area. This same amount of area will be swept every day regardless of where in its orbit the planet is.

As a planet travels in its elliptical orbit, its distance from the Sun will vary. As an equal area is swept during any period of time and since the distance from a planet to its orbiting star varies, one can conclude that in order for the area being swept to remain constant, a planet must vary in speed.

The physical meaning of the law is that the planet moves faster when it is closer to the sun. This is because the sun's gravity accelerates the planet as it falls toward the sun, and decelerates it on the way back out.

The actual speeds are:

Maximum orbital speed at perihelion: 30.287 km/s (109,033 km/h)
Average orbital speed: 29.783 km/s (107,218 km/h)
Minimum orbital speed at aphelion: 29.291 km/s
(105,448 km/h)

And the distances:

Aphelion 152,097,701 km (1.016 AU)
Perihelion 147,098,074 km (0.983 AU)
Semi-major axis 149,597,887 km
Semi-minor axis 149,576,999 km
Orbital circumference 924,375,700 km (6.179 AU)

Answer 2

Yes, Earth's speed varies from 29.29 km/s at aphelion to 30.29 km/s at perihelion. But northern hemisphere winter occurs at perihelion, not aphelion. That is because then seasons are not caused by the changing distance to the Sun, the difference is too small. The seasons are caused by the changing angle at which the Sun's rays hit Earth due to the Earth's axis being tilted 23 degrees to the plane of its orbit. This also explains how it can be winter in Europe on the same day that it is summer in Australia.

Answer 3

Velocity Of Earth Around Sun

Answer 4

Actually you need to check the facts of your premise if your talking about the nothern hemisphere. The earth is closer to the sun in winter and farther in the summer. The axial tilt of the earth determines the season, NOT the distance, which only varies by a few percent.

Newton's law only applies regarding the balace of forces only applies to perfectly circular orbits and the Earth is not perfectly circular. However, your basic idea is on the right track. The earth does gain velocity when it's closest to the sun and loses it when it's farthest away. So over the course of one orbit the forces do balance out

Answer 5

I think Juniper is the closest in this answer if not right on. The law of equal area, in an eliptic orbit, the area covered by the orbiting body from point a to point be for a CERTAIN time must equal the same area point c to point d for the same CERTAIN time. Draw a line from body at A (a point far away from Focus) to Focus (sun). Go certain time and draw a line from body to focus. This will form a triangular shaped area a certain coverage size.

Move body closer to the focal point and repeat the line drawing. The height of the area will be shorter because the body is closer, but c and d will be farther apart for the same time. But the areas of the two will be the same.

Answer 6

Dude, calm down.

All planets travel around the sun in ellptical orbits. Because of this, they rotate about the sun with varying LINEAR speed. Their angular speed remains the same.

Think of the orbit of a comet--or even Pluto. Both are very elliptical in shape, and therefore their speeds when furthest from the sun as compared to when they're closest to the sun vary significantly. When an object in its orbit is closest to the sun, just think of the reason why it obtains a faster speed as being due to the fact that it has to work harder to resist the gravitational pull of the sun on it. It's linear speed increases to counter falling into the sun. When further away, it doesn't have to work nearly as hard to resist the sun's gravitational pull, and thusly rotates at a lesser linear speed in its orbit.

There is much more on this, but I don't have my space science notes with me (sorry), though when I do obtain them, I'll come back and edit my answer for you.

Don't make things more complicated than they need me; you'll only frutstrate yourself... =\

Answer 7

You've almost got it. Yes, it is true that for an orbiting body centrifugal and centripetal forces have to be equal to achieve a circular orbit. But the Earth is not in a circular orbit.

Earth's orbit is elliptic, but very slightly so. It's eccentricity is 0.0167. The maximum orbital speed is 30.287km/s (67,764 mph) while its minimum speed is 29.291km/s (65,536 mph).

There is a good picture on this site of earth's orbit:
http://www.crrel.usace.army.mil/permafrosttunnel/1g1a_Ice_Age_History.htm

One thing you may notice is that during "winter", Earth is closer to the sun than "summer." The distance to the sun has very little effect on the seasons, in comparison to the axial tilt of the Earth.

Answer 8

Juniper seems to have got it all sewn up, so I will just comment on Kepler and Newton.

Copernicus proposed a heliocentric model of the Solar System in 1543,

Kepler used that model when he derived his three laws of planetary motion based on Tycho Brahe's observations, Brahe was exceptionally keenly-sighted, Kepler almost blind. They worled together in Prague,

He published his results in his works Astronomia nova, Harmonice Mundi (1619) and the textbook Epitome of Copernican Astronomy.

Using these laws, he was the first astronomer to successfully predict a transit of Venus (for the year 1631). Kepler's laws were the first clear evidence in favour of the heliocentric model of the solar system, because they only came out to be so simple under the heliocentric assumption.

Kepler, however, never discovered the deeper reasons for the laws. Isaac Newton eventually showed that the laws were a consequence of his laws of motion and law of universal gravitation.

So whilst Kepler's Laws can be perceived as being a consequence of Newton's Laws, they actually came first, historically and were based on inferences from Brahe's empirical observations, Newton later supplied the theoretical underpinning for Kepler's Laws.

That is all I wanted to say,

Answer 9

earth does not actually moves in a circle, but in an ellipse(ovalised shape) with sun at one of its focii. it coveres equal area (on the plane of its movement) in equal time periods...
Hence we cant associate the term VELOCITY to its motion...rater we can say about the speed which obiviously is a function of time and distance from the sun. ;-)

Answer 10

Yes, it moves slowest at aphelion and fastest at perihelion.