# Argument of the Perihelion

1. Jul 4, 2008

### Philosophaie

I have seen conflicting values for the angle where the Argument of the Perihelion of the Earth or where the Winter Solstice(Dec 21) strikes. This is where the ellipse value of the True Anomaly is zero. The two values are: w=114.20783 @J2000 or w=282.9404*4.70935e-5*d where d is a formula encompassing the Julian Date. These values have different Longitude of the Ascending Node by that is just where all the planets are referenced at and does not change the Argument of the Perihelion. My question is: which is the correct Argument of the Perihelion of the Earth?

2. Jul 6, 2008

### tony873004

Normally, orbital elements are given with respect to the ecliptic plane. Argument of perihelion is the amount of degrees that seperates the longitude of perihelion from the longitude of the ascending node. But Earth's orbit by definition has no inclination since Earth's orbit defines the ecliptic. Therefore, it has no ascending node.

In reality, Earth does have a little bit of inclination since inclination is an oscillating orbital element, we can only define it at 0 for an instant in time, which was Jan 1, 2000. Although it has drifted a small amount since then, it is still effectively 0, meaning that the ascending node is weakly defined, and can rapidly change.

Winter solstice has nothing to do with perihelion. It's when Earth's tilt is directed away from the Sun from the Northern hemisphere.

3. Jul 6, 2008

### Staff: Mentor

The winter solstice does not occur at perihelion, and neither value is constant. The Earth's axial tilt changes largely because of lunisolar precession. The time between solstice and perihelion passage changes because of anomalistic precession.

Note that longitude of ascending node is not a particularly well-defined concept for the Earth's orbit, and hence neither is the argument of perihelion particularly well-defined. Astronomers prefer to use longitude of perihelion when describing the Earth's orbit.

A couple preliminary questions: Where did you get these values? Are you sure you are interpreting them correctly? Regarding your values: One value is specific to a particular epoch and to that epoch only. The other is a more general expression that represents the time variability as a linear function of time. Both might well be correct; post your sources.