Zon Mosaic Pro5 Guide de l'utilisateur télécharger pdf (Page 164)

164 Starry Night User’s Guide

eccentricity of an object in a fixed orbit

has a value between 0 and 1. The larger

an object’s eccentricity, the more its

distance from its parent body varies

throughout its orbit. Pluto has the

largest eccentricity of any planet in our

solar system. Objects in relatively

circular orbits have eccentricities near

0, while objects in extremely elliptical

orbits (such as many comets), have

eccentricities close to 1.

• Pericentre distance (q): The distance

between the object and its parent body

at the point in the object’s orbit where it

is closest to the parent. Taken together,

pericentre distance and eccentricity

specify the distance between an object

and its parent body at any point in the

object’s orbit. Pericentre distance is

given in units of AU (astronomical

units). For objects in orbit about the

Sun, such as comets, pericentre

distance is also called

perihelion

distance

.

• Ascending Node: This angular

measurement specifies the point at

which the orbit crosses northward

through the orbit’s reference plane

(references planes are described in

“Reference Plane

” on page 166),

relative to the Prime Meridian of the

parent body.

• Argument of Pericentre: Recall that the

pericentre is the point in an object’s

orbit when the object is closest to the

parent body. The Argument of

Pericentre specifies the angular

location of the pericentre, measured in

degrees.

This value is determined by measuring

the angle between the ascending node

and the pericentre, as measured from

the centre of the parent body. For

example, when the Argument of

Pericentre is 0

°, the pericentre occurs

at the same place in an object’s orbit as

the ascending node. That means that

the object would be closest to its parent

body just as it rises up through the

reference plane. Likewise, when the

Argument of Pericentre is 180

°, the

object, as it rises up through the

reference plane, is at its farthest from

its parent.

• Inclination (i): The orbit’s elliptical

shape lies in a plane known as the

orbital plane. Inclination is the angle

between the orbital plane and the

reference plane, measured between 0°

and 180°. Assume the reference plane

is the parent body’s equatorial plane. In

this case, if the object’s orbit lies in this

plane, the inclination is 0°, and the

object circles the parent body’s equator.

At an inclination of 90°, the orbit is

perpendicular to the parent body’s

equator, and the object passes over the

parent body’s north and south poles. An

inclination of greater than 90°

describes a retrograde orbit.

Cool fact: Spy satellites often have orbits

with inclinations close to 90°. From this

inclination, they can examine all parts of

Earth as it rotates beneath them.

• Pericentre Time: The preceding five

orbital elements are sufficient to

describe the orbit of a new object.

However, a value for pericentre time is

needed to determine an object’s place

on this orbital path at any time. The

pericentre time is the time (expressed

as a Julian day) when the object is at

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