# What Do Epoch J2000.0 Orbital Elements Tell Us About Earth's Orbit?

• mattrix
In summary, Epoch J2000.0 refers coordinate systems to the mean equinox and mean ecliptic of January 1, 2000, noon TT. I have the following mean orbital elements for Earth with element date January 1, 2000 and referred to j2000.0 epoch. a - semi-major axis e - eccentricity p - longitude of perihelion L - longitude of planet O - longitude of ascending node, of the intersection of the orbital plane and the plane of the ecliptic i - inclination, angle between the plane of the ecliptic and the plane of orbit Longitude
mattrix

Epoch J2000.0 refers coordinate systems to the mean equinox and mean ecliptic of January 1, 2000, noon TT.
I have the following mean orbital elements for Earth with element date January 1, 2000 and referred to j2000.0 epoch.

a := 1.00000011 - 0.00000005 *cy
e := 0.01671022 - 0.00003804 *cy
p := 102.94719 + 1198.28/3600 *cy
L := 100.46435 + 129597740.63/3600 *cy
i := 0.00005 - 46.94/3600 *cy
O := -11.26064 - 18228.25/3600 *cy

a - semi-major axis
e - eccentricity
p - longitude of perihelion
L - longitude of planet
O - longitude of ascending node, of the intersection of the orbital plane and the plane of the ecliptic
i - inclination, angle between the plane of the ecliptic and the plane of orbit

as I understand it,
the Earth's orbital plane is synonomous with the ecliptic and
as these planes do not intersect the zero direction is taken to be the vernal equinox

I realize that these references move over time, hense epoch.

However I'm confused about how at the time of epoch the last 2 elements can have a value, not equal to zero?
Where is "O" measured from, and to?
I'm not sure about the use of "longitude" either, is this figurative or literal for these 3 elements?

thanks matt

PS does anyone have the Osculating elements for the solar system plants at January 1, 2000.

Earth's inclination is not fixed. Like the other planets, its inclination changes. The j2000.0 epoch will define the ecliptical plane as the inclination of the Earth/Moon barycenter on January 1, 2000 at noon. Here's a graph I made with Gravity Simulator showing the inclination of the Earth for a few decades surrounding 2000. Note that it doesn't completely zero-out at j2000. My guesses are that this graph is Earth's instantaneous inclination, rather than the inclination of the EM barycenter. And it's the inclination with respect to the Sun rather than with respect to the SS barycenter. But those are just my guesses.

If inclination is exactly 0, longitude of ascending node is undefined. I believe that in this instance, it's specifically defined as the vernal equinox. In the real universe, there's no such thing as an inclination of exactly 0, except for the instantaneous moment when you define the plane. As the graph shows, it immediately drifts.

"Longitude" means degrees from the vernal equinox. "Argument" means degrees away from the longitude of longitude of ascending node.

You can use JPL's Horizons system to generate the orbital elements for any solar system object.
http://ssd.jpl.nasa.gov/?horizons

Hi Tony,

tony873004 said:
If inclination is exactly 0, longitude of ascending node is undefined. I believe that in this instance, it's specifically defined as the vernal equinox. In the real universe, there's no such thing as an inclination of exactly 0, except for the instantaneous moment when you define the plane.

Thats my point, these data are for the exact same moment as when the J2000 plane is defined, and they are both "mean" values.

So if there is an inclination at this time, what is the plane its inclined to?

Interesting graph, if your guesses are right, then over these few decades the average inclination of the Earth is always greater than its mean!

matt

I think the OP's confusion stems from a typo:

O - longitude of ascending node, of the intersection of the ORBITAL plane and the plane of the ecliptic

O - longitude of ascending node, of the intersection of the EQUATORIAL plane and the plane of the ecliptic

the J2000.0 Obliquity of the Ecliptic is 23° 26' 21.406"

## What are coordinates and orbits?

Coordinates refer to a set of numbers or values that are used to locate a specific point or object in space. Orbits, on the other hand, refer to the path followed by a celestial body as it moves around another object due to gravitational force.

## Why do we use coordinates and orbits in studying space?

Coordinates and orbits are essential tools in understanding and predicting the movements of objects in space. They allow us to track the positions of celestial bodies and determine their paths, which helps us in studying their behavior, interactions, and other important characteristics.

## How do we determine the coordinates and orbits of celestial bodies?

The coordinates of a celestial body can be determined using the equatorial coordinate system, which uses the celestial equator and the celestial poles as reference points. The orbits of celestial bodies can be calculated using Kepler's laws of planetary motion, which describe the relationship between the distance, speed, and period of an object in orbit around another object.

## What are some factors that can affect the coordinates and orbits of celestial bodies?

The coordinates and orbits of celestial bodies can be affected by various factors such as the gravitational pull of other objects, the shape of their orbit, and the presence of other planets or moons. These factors can cause changes in the speed, direction, and shape of an object's orbit.

## What are some real-world applications of understanding coordinates and orbits?

Understanding coordinates and orbits is crucial in space exploration, satellite communication and navigation, and astronomy. It also helps in predicting and preventing potential collisions between celestial bodies and in studying the effects of gravitational forces on objects in space.