# 10: Computation of an Ephemeris

- Page ID
- 6853

- 10.1: Introduction to an Ephemeris
- The entire enterprise of determining the orbits of planets, asteroids and comets is quite a large one, involving several stages. From the available observations, the orbit of the body has to be determined; in particular we have to determine the orbital elements, a set of parameters that describe the orbit. For a new body, one determines preliminary elements from the initial few observations that have been obtained. As more observations are accumulated, so will the calculated preliminary elements

- 10.5: Elements of a Parabolic Orbit
- The eccentricity, of course, is unity, so only five elements are necessary. In place of the semi major axis, one usually specifies the parabola by the perihelion distance q . Presumably no orbit is ever exactly parabolic, which implies an eccentricity of exactly one. However, many long-distance comets move in large and eccentric orbits, and we see them over such a short arc near to perihelion that it is not possible to calculate accurate elliptic orbits, and we usually then fit a parabolic orbi

- 10.7: Calculating the Position of a Comet or Asteroid
- We suppose that we are given the orbital elements of an asteroid or comet. Our task is to be able to predict, from these, the right ascension and declination of the object in the sky at some specified future (or past) date. If we can do it for one date, we can do it for many dates - e.g. every day for a year if need be. In other words, we will have constructed an ephemeris.