Home > astronomy > Comet Lovejoy flies close enough to Earth for naked-eye view

Comet Lovejoy flies close enough to Earth for naked-eye view

This photo of Comet C/2014 Q2 Lovejoy was taken in late December 2014 at an approximate magnitude of 5.6. Photo courtesy Wikimedia, photo by Paul Stewart.

This photo of Comet C/2014 Q2 Lovejoy was taken in late December 2014 at an approximate magnitude of 5.6. Photo courtesy Wikimedia, photo by Paul Stewart.

Earthlings had their last chance for 8,000 years to see Comet Lovejoy Saturday night.

The opportunity to look up and see its green glow came when the comet was six days away from perihelion. On Jan. 30 the comet, formally known as Comet C/2014 Q2 Lovejoy, will be 193 million kilometers from the sun.

The comet actually passed closest to Earth on Jan. 7, when it was 70.2 million kilometers away.

It will be about 13,000 years before Comet Lovejoy is again close enough to Earth to be seen without the aid of a magnifying device.

How do scientists know how long it has been since a comet was visible with only the eyes from the surface of our planet?

The answer involves mathematics. Among the laws of planetary motion discovered by Johannes Kepler is the equation necessary to determine a comet’s period, or the amount of time needed to complete an orbit, if its semi-major axis is known. The semi-major axis of an ellipse, which is the shape that most objects in orbit around the sun follow, is best thought of as the radius of the ellipse at its two points that are most distant from each other.

This graphic shows the semi-major and semi-minor axes of an ellipse. Courtesy Wikimedia.

This graphic shows the semi-major and semi-minor axes of an ellipse. Courtesy Wikimedia.

Kepler determined that the square of the ellipse’s period is equal to the cube of the semi-major axis’ length.

Another interesting feature of Comet Lovejoy’s orbit is its orientation relative to the plane of Earth’s and the other planets’ orbits. As Phil Plait explained at Bad Astronomy, the comet sits about 80 degrees off those trajectories.

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