Astronomy Answers
AstronomyAnswerBook: Motion of the Earth

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1. The Orbit of the Earth ... 2. Measuring Motion

This page answers questions about motion, including motion of you and the Earth through the Universe. The questions are:


Your path through the Universe is not a straight line, but rather a complicated curved trajectory with at least the following contributions:

  1. Your speed with respect to the surface of the Earth. I do not know what this speed is. It is not constant, is probably usually smaller than 150 km/h (100 mph), and changes with a certain regularity but also with irregular deviations.
  2. The speed at which your location on Earth rotates around the axis of the Earth. The Earth has a diameter of about 12,800 km (7,900 miles) and rotates around its axis (relative to the stars: its sidereal rotation period) once every 23 hours and 56 minutes. The speed depends on your position on Earth. At the equator, the Earth rotates at a speed of 1670 km/h or 0.46 km/s (1,040 mph or 0.29 miles per second). Away from the equator, the Earth's rotation speed decreases, and it is zero at the poles. At 51 degrees latitude (north or south, it doesn't matter which), the rotation speed is about 1050 km/h. The direction of this speed rotates over 360 degrees in about a day, and is at every time in the opposite direction from what it was about 12 hours ago.
  3. [303]

  4. The speed at which the Earth orbits around the common barycenter with the Moon. The Moon orbits around the Earth at an average distance of 384,400 km in (relative to the stars) 27.32 days, at a speed of on average 1.0 km/s (3600 km/h). The gravity of the Moon attracts the Earth, just like the gravity of the Earth attracts the Moon, and because of this both the Earth and the Moon orbit around their common center of mass. Because the Earth has a lot more mass than the Moon, the orbit of the Earth around the common center of mass is far smaller than the orbit of the Moon. The speed of the Earth around the common barycenter is about 45 km/h, and its direction changes by 360 degrees in about a month.
  5. The speed at which the Earth and the Moon together orbit around the center of mass of the Solar System (which is almost the same as the center of the Sun). The Earth and Moon orbit around the Sun once every 365.24 days, at an average distance of 1 AU (150 million km) and an average speed of 29.8 km/s or (107,000 km/h). That speed is at each moment in the opposite direction from what it was about 6 months before. The speed varies during a year by about 3 percent (faster in January, slower in July).
  6. The speed at which the Sun (and the rest of the Solar System) move relative to the average of the nearby stars. This speed is at the moment about 20 km/s or 72,000 km/h, but can change in the future because of the gravity of other stars that the Sun may pass on its travels.
  7. [87]

  8. The speed at which the Solar System and nearby stars move around the center of the Milky Way. The Sun is about 30,000 lightyears from the center of the Milky Way and orbits around that center in about 200 million years, with an average speed of about 230 km/s or 800,000 km/h. The Sun has completed about 23 of those orbits so far. Our Milky Way is a spiral galaxy with a diameter of about 100,000 lightyears and seems to contain at least four spiral arms. The Sun lies between the Sagittarius-Carina arm and the Perseus arm.
  9. [16]

  10. The speed at which our Milky Way moves relative to the center of gravity of the Local Group of galaxies. The Local Group has a diameter of about 3 million lightyears and contains two big galaxies and about 20 smaller ones. The two big galaxies are our own Galaxy and the Andromeda Nebula (M 31) at a distance of 2.2 million lightyears. The Andromeda Nebula can be seen from the northern hemisphere of Earth. Some of the smaller galaxies close to the Andromeda Nebula are M 32, M 33, and NGC 205. Some of the smaller galaxies close to our own Galaxy are the Large and Small Magellanic Clouds (at about 150,000 lightyears distance), which can be seen from the southern hemisphere of Earth. Our Galaxy moves toward the center of the Local Group at a speed of about 40 km/s (144,000 km/h). This speed can change in the future, under the influence of the gravity of mostly the Andromeda Nebula.
  11. The speed of the Local Group compared to the cosmic microwave background (CMB). A very small part of the radiation that was generated just after the Big Bang can be detected today in all directions, and looks like the thermal radiation that comes from something at a temperature of 3 kelvin, with most power in the microwave part of the electromagnetic spectrum. This CMB is almost equally strong in all directions, except for a small deviation that is best explained as result of the doppler effect due to the motion of the Sun relative to the CMB. The speed of the Sun relative to the CMB is now 369 km/s or 1,328,000 km/h (according to // From this speed one can estimate the speed of the Local Group relative to the CMB. Some estimate this at about 600 km/s (2,200,000 km/h), but this speed can change in the future under influence of the gravity of the other groups of galaxies that might pass close by the Local Group.

All of these speeds are in different directions. The motions that keep going around (numbers 2, 3, 4, and 6) do that each at a different period and in a different plane.

The sum of all of these speeds (each in its own direction) yields the speed relative to the CMB, which I think is the best measure for "your speed relative to the Universe". The speed of the Sun relative to the CMB is now 1,328,000 km/h (plus or minus 9000 km/h) in the direction of the constellation of the Cup (just south of the Lion). Contributions 2 - 4 make your speed relative to the CMB up to about 105,000 km/h greater (greatest around 15 December, if my calculations are correct) or smaller (least around 12 June) than the speed of the Sun relative to the CMB, mostly depending on the season.

The direction in which the Sun moves relative to the CMB has (relative to equinox J2000.0) a right ascension of 11h11m57s (± 23s) and declination −7.22° (± 0.08°), according to the publication I mentioned above. According to my calculations, the direction in which the Earth moves relative to the CMB can vary during a year between right ascension 10h55m and 11h29m, and between declinarion −9.3° and −5.3°.

The Sun has a diameter of 1,392,000 km and does not rotate around its axis in the same time everywhere. Near the solar equator, the material goes around the rotation axis (measured relative to the stars) once every 25 days, but near the poles the material goes around once every 34 days.

The Local Group of galaxies is a member of a much larger collection of galaxies which is known as the Local Supercluster. The Virgo Cluster is the main member of the Local Supercluster and is about 45 million lightyears away from us. Our Local Group of galaxies moves at a speed of about 600 km/s (2.2 million km/h) relative to the universe as a whole, but this speed is not directed straight toward the center of the Local Supercluster, so astronomers think there must be another very massive collection of galaxies beyond the Local Supercluster. This surmised collection is known as the Great Attractor, but its exact location or nature is not known.


1. The Orbit of the Earth

The orbit of the Earth relative to the Sun is slightly different each year, mostly because the gravity of the other planets pulls the Earth a bit closer to themselves, and the locations of the planets are different each year. Even with these perturbations, the shape of the orbit of the Earth looks very much like an ellipse, and next year's ellipse is very close to this year's ellipse. The exact orbit that the Earth follows is not any well-known geometrical shape, and it is not possible to describe that exact orbit in just a few words.

I calculated the position of the Earth relative to the Sun for 100,000 times between the years 1500 and 2500, using the VSOP model, and find that the Earth is at any time on average 10,500 km away from where it was exactly one orbit ago. That distance is less than the diameter of the Earth, and only about 0.001 % of the length of the orbit of the Earth.


2. Measuring Motion

Astronomers can measure whether an object in space comes toward Earth or moves away from Earth. The Doppler effect allows them to measure this. The Doppler effect causes waves (like light waves) that come from some object to have a higher frequency when the object is coming toward us and a lower frequency when the object moves away from us. If you know what the frequency is for an object that does not move relative to us, and if you measure the frequency for the object of interest, then you can determine from the difference how fast the object moves along the line toward us. This way we can measure the speed along the line towards Earth, but not the speed in directions that go away from that line.



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Last updated: 2017-04-24