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)

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Sky Highlights - September 2017

Opposition
Neptune reaches opposition on September 5th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for September

Northern Hemisphere
Evening
West:- Jupiter (mag. -1.7)
Southwest:- Saturn (mag. +0.5)
East:- Neptune (mag. +7.8)
Midnight
South:- Neptune
East:- Uranus (mag. +5.7)
Morning
West:- Neptune
South:- Uranus
East:- Venus (mag. -3.9), Mars (mag. +1.8) (from second week), Mercury (mag. +0.5 to -1.3) (from second week)

Southern Hemisphere
Evening
West:- Jupiter
Northwest:- Saturn
East:- Neptune
Midnight
West:- Saturn
North:- Neptune
Northeast:- Uranus
Morning
West:- Neptune
Northwest:- Uranus
Northeast:- Venus
East:- Mars (end of month)

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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