What ecliptic longitudes actually tell you

A planet's ecliptic longitude is the main number in any astrology table and, at the same time, the most frequently misread one. Here is what it means and why you cannot infer visibility from it directly.

What the longitude measures

The ecliptic is the imaginary circle on the sky that is the projection of the plane of Earth's orbit. A planet's longitude is measured along it, in degrees from the vernal equinox. Just like longitude on Earth it is cyclic: 370° is 10°.

There are two reference frames and they are easy to confuse. The geocentric longitude is the angle at which the planet appears from Earth. The heliocentric longitude is its position on its own orbit relative to the Sun. For the outer planets the two are close together; for the inner ones they can be far apart.

What else you need besides longitude

Longitude does not determine whether you will see the planet. Three more things are needed:

Declination and altitude. A planet can have a perfectly "zodiacal" longitude and still be below the horizon at your latitude. Visibility is set by altitude above the horizon, which depends on the observer's latitude.

Angular elongation from the Sun. This is the key to visibility. Venus and Mercury rarely move more than 30–47° from the Sun and so almost always vanish into its glare. Jupiter and Saturn sit far away and stay visible for months.

Magnitude. Brightness. −4 for Venus, +1 for Saturn, +6 for Uranus — the same Uranus can be an obvious point or barely detectable depending on phase and distance.

Why the motion looks uneven

Planets orbit at different speeds and Earth is moving too. Usually a planet appears to advance directly, but when Earth overtakes it the projection starts moving backwards. That is the retrograde stretch. It does not mean the orbit changed: the planet goes on along its way.

Reading the city table

The longitude in the table is geocentric. Beside it sits the zodiac sign computed from it in the chosen system, which is exactly why the sign differs in the sidereal variant from the tropical one: the reference point is shifted. The number moves slowly, but the sign changes the moment it crosses a boundary.