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 - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
Evening
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
Midnight
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Morning
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
Evening
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
Midnight
West:- Jupiter
North:- Saturn
East:- Neptune
Morning
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

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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