Orbit in 3 minutes: a, e, i in simple terms

When you search an asteroid name, you’ll often see three symbols: a, e, and i. Here’s what they mean without the math overload.

a — Semi-major axis (orbit size)

• a is the average size of the orbit.
• It’s measured in AU (astronomical units).
• 1 AU ≈ 149,597,870 km (Earth–Sun distance).

If an asteroid has a ≈ 2.5 AU, it usually lives in the main belt between Mars and Jupiter.

e — Eccentricity (orbit shape)

• e = 0 is a circle.
• Higher e means a more elongated ellipse.
• Most main-belt asteroids have moderate eccentricities.

i — Inclination (orbit tilt)

• i is the tilt relative to Earth’s orbital plane.
• 0° means “flat” in the same plane.
• Higher values mean the orbit is more tilted.

Why we show a mini-visual

Our mini-visual is a schematic, not a scale model. It’s a quick intuition builder: bigger a means a bigger orbit; bigger e means a more stretched ellipse.

Keyword focus: “what is eccentricity”, “semi major axis explained”, “inclination orbit meaning”.

Next: close approaches

If the object has close-approach data, we also show:

• distance in AU + km
• relative velocity

Read more: /en/blog/close-approaches-explained