Question

why halley's comet comes after every 72 years only?why only after 72 years why not 73years?

Answer 1

Halley age is 72 years

Answer 2

Halley's orbital period has varied between 74–79 years since 240 BC.[28][11] Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967 (with 0 being a circle and 1 being a parabolic trajectory). The perihelion, the point in the comet's orbit when it is nearest the Sun, is just 0.6 AU.[33] This is between the orbits of Mercury and Venus. Its aphelion, or farthest distance from the Sun, is 35 AU (roughly the distance of Pluto). Unusual for an object in the Solar System, Halley's orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or, clockwise from above the Sun's north pole. The orbit is inclined by 18° to the ecliptic, with much of it lying south of the ecliptic. (Because it is retrograde, the true inclination is 162°).[34] Due to the retrograde orbit, it has one of the highest velocities relative to the Earth of any object in the Solar System. The 1910 passage was at a relative velocityof 70.56 km/s (157,838 mph or 254,016 km/h).[35] Because its orbit comes close to Earth's in two places, Halley is associated with two meteor showers: the Eta Aquariids in early May, and the Orionids in late October.[36]Halley is the parent body to the Orionids. Observations conducted around the time of Halley's appearance in 1986 suggested that the comet could additionally perturb the Eta Aquaridmeteor shower, although it might not be the parent of that shower.[37]