The orbital eccentricity of an astronomical object is a parameter that describes the shape of the orbit. A perfectly circular orbit has an eccentricity of zero. Elliptical orbits have values between 0 and 1 with higher values indicating a more eccentric orbit. An eccentricity of exactly 1 is a parabolic orbit and anything greater than that is a hyperbolic orbit.

The orbits of the planets are almost circular with the exception of Mercury (0.206) and Mars to a lesser extent. The Earth has an eccentricity of 0.017. Most asteroids have orbital eccentricities between 0 and 0.35 with the average value equal to 0.17. The eccentricities of comets vary widely. They can be as low as 0.2 for periodic comets but often close to or greater than 1 for non-periodic comets.

Mercury has the most eccentric orbit of the planets (credit:- NASA)

Glossary A B C D E F G H I J K L M N O P Q R S T U V W X Y Z


Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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