Rotation Period

 

The Length of the Day

The most fundamental astronomical unit of time is the day, measured in terms of the rotation of Earth. There is, however, more than one way to define the day. Usually, we think of it as the rotation period of Earth with respect to the Sun, called the solar day. After all, for most people sunrise is more important than the rising time of Arcturus or some other star, so we set our clocks to some version of Sun-time. However, astronomers also use a sidereal day, which is defined in terms of the rotation period of Earth with respect to the stars.

A solar day is slightly longer than a sidereal day because (as you can see from Figure 4.10) Earth not only turns but also moves along its path around the Sun in a day. Suppose we start when Earth’s orbital position is at day 1, with both the Sun and some distant star (located in the direction indicated by the long white arrow pointing left), directly in line with the zenith for the observer on Earth. When Earth has completed one rotation with respect to the distant star and is at day 2, the long arrow again points to the same distant star. However, notice that because of the movement of Earth along its orbit from day 1 to 2, the Sun has not yet reached a position above the observer. To complete a solar day, Earth must rotate an additional amount, equal to 1/365 of a full turn. The time required for this extra rotation is 1/365 of a day, or about 4 minutes. So the solar day is about 4 minutes longer than the sidereal day. Figure 4.10 Difference Between a Sidereal Day and a Solar Day. This is a top view, looking down as Earth orbits the Sun. Because Earth moves around the Sun (roughly 1° per day), after one complete rotation of Earth relative to the stars, we do not see the Sun in the same position.

Because our ordinary clocks are set to solar time, stars rise 4 minutes earlier each day. Astronomers prefer sidereal time for planning their observations because in that system, a star rises at the same time every day.

3 Orbits and Gravity. Key Terms

 

angular momentum

the measure of the motion of a rotating object in terms of its speed and how widely the object’s mass is distributed around its axis

aphelion

the point in its orbit where a planet (or other orbiting object) is farthest from the Sun

apogee

the point in its orbit where an Earth satellite is farthest from Earth

asteroid belt

the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun

astronomical unit (AU)

the unit of length defined as the average distance between Earth and the Sun; this distance is about 1.5 × 108 kilometers

density

the ratio of the mass of an object to its volume

eccentricity

in an ellipse, the ratio of the distance between the foci to the major axis

ellipse

a closed curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same

escape speed

the speed a body must achieve to break away from the gravity of another body

focus

(plural: foci) one of two fixed points inside an ellipse from which the sum of the distances to any point on the ellipse is constant

gravity

the mutual attraction of material bodies or particles

Kepler’s first law

each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse

Kepler’s second law

the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time

Kepler’s third law

the square of a planet’s orbital period is directly proportional to the cube of the semimajor axis of its orbit

major axis

the maximum diameter of an ellipse

mass

a measure of the amount of material within an object

momentum

the measure of the amount of motion of a body; the momentum of a body is the product of its mass and velocity; in the absence of an unbalanced force, momentum is conserved

Newton’s first law

every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force

Newton’s second law

the change of motion of a body is proportional to and in the direction of the force acting on it

Newton’s third law

for every action there is an equal and opposite reaction (or: the mutual actions of two bodies upon each other are always equal and act in opposite directions)

orbit

the path of an object that is in revolution about another object or point

orbital period (P)

the time it takes an object to travel once around the Sun

orbital speed

the speed at which an object (usually a planet) orbits around the mass of another object; in the case of a planet, the speed at which each planet moves along its ellipse

perigee

the point in its orbit where an Earth satellite is closest to Earth

perihelion

the point in its orbit where a planet (or other orbiting object) is nearest to the Sun

perturbation

a small disturbing effect on the motion or orbit of a body produced by a third body

satellite

an object that revolves around a planet

semimajor axis

half of the major axis of a conic section, such as an ellipse

velocity

the speed and direction a body is moving—for example, 44 kilometers per second toward the north galactic pole

Charon 🌑🌒🌓🌔🌕🌖🌗🌘🌑 🔭

Charon (/ˈkɛərən/ or /ˈʃærən/), known as (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).

With half the diameter and one eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Plutonian system lies outside Pluto, and the two bodies are tidally locked to each other.

The reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be essential ingredients of life. These tholins were produced from methane, nitrogen and related gases which may have been released by cryovolcanic eruptions on the moon,[21][22] or may have been transferred over 19,000 km (12,000 mi) from the atmosphere of Pluto to the orbiting moon.

The New Horizons spacecraft is the only probe that has visited the Pluto system. It approached Charon to within 27,000 km (17,000 mi) in 2015.