Semi-major axis - Revision history

Suitupandshowup: added image

2018-06-25T22:39:54Z

added image

← Older revision		Revision as of 23:39, 25 June 2018
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	[[File:Semi-major and semi-minor axes of an ellipse.jpg\|thumb\|upright=1.2\|The semi-major (''a'') and semi-minor axis (''b'') of an ellipse]]		[[File:Semi-major and semi-minor axes of an ellipse.jpg\|thumb\|upright=1.2\|The semi-major (''a'') and semi-minor axis (''b'') of an ellipse]]

	~~File:Semi-major and semi-minor axes of an ellipse.jpg~~

	==Effects of thrust==		==Effects of thrust==

	[[Prograde]] thrust (accelerating the body in the direction in which it is moving) increases the semi-major axis whereas [[retrograde]] thrust (in the opposite direction to its movement) decreases it. It is important to note that if no force acts on an orbiting body, it will return to the position where it last performed a maneuver. The opposite side of the orbit is most affected by any thrust whereas the orbit remains almost unchanged close to the point where the maneuver is performed.<br />		[[Prograde]] thrust (accelerating the body in the direction in which it is moving) increases the semi-major axis whereas [[retrograde]] thrust (in the opposite direction to its movement) decreases it. It is important to note that if no force acts on an orbiting body, it will return to the position where it last performed a maneuver. The opposite side of the orbit is most affected by any thrust whereas the orbit remains almost unchanged close to the point where the maneuver is performed.<br />
	Prograde and retorgrade thrust at either [[apsis]] of an orbit has the greatest influence on the semi-major axis, by moving the opposite apsis. This means that a spacecraft wishing to go from one circular orbit to a [[superior orbit\|superior]] or [[inferior orbit\|inferior]] one can do so most efficiently by performing a [[Hohmann transfer]] (in the absence of other forces).		Prograde and retorgrade thrust at either [[apsis]] of an orbit has the greatest influence on the semi-major axis, by moving the opposite apsis. This means that a spacecraft wishing to go from one circular orbit to a [[superior orbit\|superior]] or [[inferior orbit\|inferior]] one can do so most efficiently by performing a [[Hohmann transfer]] (in the absence of other forces).

	==Relation to energy==		==Relation to energy==

	It is important to note that, when working with [[orbital_energy\|orbital energies]], the energy is '''not''' the same as that required to hypothetically take a body to that orbit from either the surface or the center of the body it orbits. In fact, the [[specific energy]] of a satellite in elliptical orbit around the Earth is negative.<br />		It is important to note that, when working with [[orbital_energy\|orbital energies]], the energy is '''not''' the same as that required to hypothetically take a body to that orbit from either the surface or the center of the body it orbits. In fact, the [[specific energy]] of a satellite in elliptical orbit around the Earth is negative.<br />
	Since orbital mechanics only concerns itself with changes in orbital energy, the zero could be chosen arbitrarily. It is computationally most convenient to choose the value at [[escape velocity]]. This choice makes the semi-major axis inversely proportional to the specific energy and if the mass does not change also to the total orbital energy.<br />		Since orbital mechanics only concerns itself with changes in orbital energy, the zero could be chosen arbitrarily. It is computationally most convenient to choose the value at [[escape velocity]]. This choice makes the semi-major axis inversely proportional to the specific energy and if the mass does not change also to the total orbital energy.<br />

Suitupandshowup at 22:39, 25 June 2018

2018-06-25T22:39:14Z

← Older revision		Revision as of 23:39, 25 June 2018
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	For hyperbolic orbits, see [[semi-transverse axis]] instead.		For hyperbolic orbits, see [[semi-transverse axis]] instead.

	[File:Semi-major and semi-minor axes of an ellipse.jpg[\|thumb\|upright=1.2\|The semi-major (''a'') and semi-minor axis (''b'') of an ellipse]]		[[File:Semi-major and semi-minor axes of an ellipse.jpg\|thumb\|upright=1.2\|The semi-major (''a'') and semi-minor axis (''b'') of an ellipse]]

	File:Semi-major and semi-minor axes of an ellipse.jpg		File:Semi-major and semi-minor axes of an ellipse.jpg

Suitupandshowup: tried to add image

2018-06-25T22:38:48Z

tried to add image

← Older revision		Revision as of 23:38, 25 June 2018
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	It is equal to the [[semi-parameter]]: <math>p=a</math><br />		It is equal to the [[semi-parameter]]: <math>p=a</math><br />
	For hyperbolic orbits, see [[semi-transverse axis]] instead.		For hyperbolic orbits, see [[semi-transverse axis]] instead.

			[File:Semi-major and semi-minor axes of an ellipse.jpg[\|thumb\|upright=1.2\|The semi-major (''a'') and semi-minor axis (''b'') of an ellipse]]

			File:Semi-major and semi-minor axes of an ellipse.jpg

	==Effects of thrust==		==Effects of thrust==

ChristiaanK: Typo

2013-02-16T13:17:05Z

Typo

← Older revision		Revision as of 14:17, 16 February 2013
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	In orbital mechanics, the '''semi-major axis''' (symbol <math>a</math> is half the [[major axis]] (distance between [[apoapsis]] and [[periapsis]]) of an elliptical orbit. It is defined as the mean distance between either focus of the orbit and any point on the orbit and equals the radius if the orbit is circular.<br />		In orbital mechanics, the '''semi-major axis''' (symbol <math>a</math>) is half the [[major axis]] (distance between [[apoapsis]] and [[periapsis]]) of an elliptical orbit. It is defined as the mean distance between either focus of the orbit and any point on the orbit and equals the radius if the orbit is circular.<br />
	It is equal to the [[semi-parameter]]: <math>p=a</math><br />		It is equal to the [[semi-parameter]]: <math>p=a</math><br />
	For hyperbolic orbits, see [[semi-transverse axis]] instead.		For hyperbolic orbits, see [[semi-transverse axis]] instead.

ChristiaanK: Expand

2013-02-16T13:16:27Z

Expand

@@ Line 1: / Line 1: @@
-{{Stub}}
+In orbital mechanics, the '''semi-major axis''' (symbol <math>a</math> is half the [[major axis]] (distance between [[apoapsis]] and [[periapsis]]) of an elliptical orbit. It is defined as the mean distance between either focus of the orbit and any point on the orbit and equals the radius if the orbit is circular.<br />
-In orbital mechanics, the '''semi-major axis''' is half the [[major axis]] of an elliptical orbit. It is inversely proportional to [[orbital energy]] for a constant mass.
+It is equal to the [[semi-parameter]]: <math>p=a</math><br />
 [[Category:Orbital Mechanics]]

ChristiaanK: I need to watch my terminology.

2013-02-06T04:00:59Z

I need to watch my terminology.

← Older revision		Revision as of 05:00, 6 February 2013
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	{{Stub}}		{{Stub}}
	In orbital mechanics, the '''semi-major axis''' is half the [[major axis]] of an elliptical orbit. It is ~~decreases in proportio~~ to [[orbital energy]] for a constant mass.		In orbital mechanics, the '''semi-major axis''' is half the [[major axis]] of an elliptical orbit. It is inversely proportional to [[orbital energy]] for a constant mass.

	[[Category:Orbital Mechanics]]		[[Category:Orbital Mechanics]]

ChristiaanK: I meant decrease

2013-02-06T03:41:39Z

I meant decrease

← Older revision		Revision as of 04:41, 6 February 2013
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	{{Stub}}		{{Stub}}
	In orbital mechanics, the '''semi-major axis''' is half the [[major axis]] of an elliptical orbit. It is ~~increases~~ in proportio to [[orbital energy]] for a constant mass.		In orbital mechanics, the '''semi-major axis''' is half the [[major axis]] of an elliptical orbit. It is decreases in proportio to [[orbital energy]] for a constant mass.

	[[Category:Orbital Mechanics]]		[[Category:Orbital Mechanics]]

ChristiaanK: Fix

2013-02-06T03:38:04Z

Fix

← Older revision		Revision as of 04:38, 6 February 2013
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	{{Stub}}		{{Stub}}
	In orbital mechanics, the '''semi-major axis''' is half the [[major axis]] of an elliptical orbit. It is ~~proportional~~ to [[orbital energy]] for a constant mass.		In orbital mechanics, the '''semi-major axis''' is half the [[major axis]] of an elliptical orbit. It is increases in proportio to [[orbital energy]] for a constant mass.

	[[Category:Orbital Mechanics]]		[[Category:Orbital Mechanics]]

ChristiaanK: Now it's exactly right

2013-02-05T16:42:59Z

Now it's exactly right

← Older revision		Revision as of 17:42, 5 February 2013
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	{{Stub}}		{{Stub}}
	In orbital mechanics, the '''semi-major axis''' is half the [[major axis]]. It ~~monotonically increases with~~ [[orbital energy]].		In orbital mechanics, the '''semi-major axis''' is half the [[major axis]] of an elliptical orbit. It is proportional to [[orbital energy]] for a constant mass.

	[[Category:Orbital Mechanics]]		[[Category:Orbital Mechanics]]

Farred: correction as per discussion

2013-02-04T18:31:26Z

correction as per discussion

← Older revision		Revision as of 19:31, 4 February 2013
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	{{Stub}}		{{Stub}}
	In orbital mechanics, the '''semi-major axis''' is half the [[major axis]]. It ~~is linearly related to the~~ [[orbital energy]].		In orbital mechanics, the '''semi-major axis''' is half the [[major axis]]. It monotonically increases with [[orbital energy]].

	[[Category:Orbital Mechanics]]		[[Category:Orbital Mechanics]]