Question

When is the tail of a comet the longest?

Answer 1

A comet's visible tail is longest as it rounds the sun.

Explanation:

A comet is a dirty ball of rock, dust, and ices (dry ice, water ice, and frozen gasses like methane). Its orbit is highly elliptical, and it warms up as it's orbit brings it closer to the sun. As it warms, water, gas and dust evaporate off the icy ball, creating a cloud around it called the coma .
The coma is hit by the solar wind - radiation and particles blasted away from the sun, which drags it out away from the comet.
The comet is most productive when it is closest to the sun, where the solar wind is also strongest.
The visible tail may continue to grow after it has rounded the sun, but the activity will decrease the farther it gets.
Fun facts:
The tail usually consists of two parts - a dust tail and an ion tail. The dust tail is made of larger particles and is brighter and often carved after the comet rounds the sun.
The tail always points away from the sun.
The stream of dust released by the comet continues to orbit the sun along the same path, and if the earth moves through that stream of dust, we get a meteor shower!