Motions of the Earth and Their Effects

  Rotation of the Earth

  • What is it? The Earth's spin around its axis, which is actually tilted, by 23.5°! How do we know? Does the Sun always really rise in the East and set in the West, and is it exactly overhead at noon? [The angle changes with a 40,000 yr cycle, possibly related to ice ages.]
  • Main effects: Day and night; Apparent rotation of the celestial sphere.
  • How long does it take? One full rotation takes about 23h 56m. [The place on the surface where we are is moving at 850 mph or so, in addition to the motion of the whole Earth.] Why is this "sidereal day" different from the 24-hour solar day?
  • [How do we know? Direct proof that the Earth turns came in 1851 with Foucault's pendulum; Also shown by the Coriolis effect, which causes hurricanes to swirl and the path of flying objects to curve.]
  • A twist - Precession of the rotation axis: The Earth "wobbles", and a cycle takes about 26,000 years; This implies that in a few thousand years Polaris will no longer be our North Star [and also that there is a precession of the equinoxes].

Revolution of the Earth

  • What is it? Our orbital motion around the Sun on the ecliptic plane, in the same direction as the rotation, at a distance of 1 astronomical unit (AU), or 150 million km [at 60,000 mph!] Takes about 365.25 days... Why don't we feel this motion?
  • Changing sky: Stars and constellations rise 4 minutes earlier every day, which leads to a gradual change of the night sky throughout the year.
  • Seasons: Cold winters and warm summers are not due to varying distance from the Sun (seasons are opposite in the northern and southern hemispheres!), but to the 23.5 tilt of Earth's axis of rotation.
  • Important dates: The Winter and Summer Solstices are, in the northern hemisphere, the shortest and longest day (but not the coldest and the hottest); on the Spring and Fall Equinoxes the day and night are equally long,* and the Sun rises exactly in the east, sets exactly in the west.
  • [What is a year, really? Tropical vs sidereal year; Why are they different?]

* In reality, because of the atmosphere, the day is a few minutes longer!

A twist - Shape of the Earth's orbit: The distance to the Sun actually varies between a minimum at perihelion [147.5 Mkm, on Jan 4th] and a maximum [152.6 Mkm].

  Use: Parallax

  • Parallax: The apparent displacement of an object with respect to the background when the observer's point of view changes.
  • Which objects? The nearer an object is compared to the baseline, the larger the angle will be; Planets, comets and asteroids move more than stars; Nearby stars (using a telescope) more than distant ones.
  • Application: Can be used to find the distance to the object if the baseline and the angles involved are known (the longer the baseline, the smaller the errors). For nearby objects (like comets, NEO's, planets), use different points on the Earth's surface; for stars, use the diameter of the Earth's orbit.
  • Historical significance: Distances to stars cannot be found this way without a telescope, which helped support the idea that the Earth does not move; Was used however to establish that comets and supernovae are way beyond the Moon.

Astronomy and Timekeeping

  • Small time units: Seconds, minutes, and hours are not natural units in astronomy, but the use of the number 60 and related ones like 12 may be indirectly related, and those units are appropriate for measuring relatively short times on human scales.
  • Day and time of day: The solar day is the one used for our timekeeping (although the sidereal day would be the more meaningful one in astronomy); To find out the time of day, use the Sun's position with respect to your meridian, for example with a sundial, which gives the apparent solar time (as opposed to mean solar time, standard time, or universal time).
  • Weeks and months: Aside from biblical reasons, the 7-day week may be related to the 7 wandering celestial objects known in antiquity, 5 planets, the Sun, and the Moon; The month comes from the length of the Moon cycle (the "synodic month", later stretched out so that a whole number of them-12!-fits in a year).
  • Year and time of year: In our calendars we use the tropical year, because of its relation to seasons; To find out the time of year, use the location where the Sun rises on the horizon, for example by marking special directions on the ground or using the alignment of buildings.
  • [Finding out where you are: For orientation, at night locate Polaris to find the North direction, during the day use the Sun together with knowledge of time of year. For latitude, see how high Polaris is on your horizon (actually can be done with any star); For longitude, much more complicated.]

page by luca bombelli <bombelli at>, modified 29 sep 2012