Understanding Orbits
Assessment Questions
11-10-25
Morehead State University
Morehead, KY
Prof. Bob Twiggs
RJTwiggs@gmail.com
1
Understanding Orbits
Sun – Earth System
Earth’s rotation axis
Equator
Eclipse
Sun’s rays
Spacecraft orbit plane
Orbiting Spacecraft
1. Why are the suns rays assumed parallel when reaching the earth?
Sun is far away and large that the rays can be assumed parallel.
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Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
2.
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How much is the earth tilted on it’s axis?
23.5 degrees
Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
3.
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Yes
Does the earth have the axis tilted all year long?
Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
4.
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What is the nadir point of a spacecraft?
The point directly below the SC toward the center of gravity.
Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
5. How is the inclination of a LEO spacecraft orbit defined?
Angle between equator and orbit plane for an ascending node.
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Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
6.
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Why is inclination defined only for an ascending node (nadir point
crossing the equator?
Need to have a unique reference. Ascending node make it unique.
Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
What is the approx. inclination of the orbit shown?
30 degrees
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Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
7.
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If a spacecraft could change directions of flight by 180 degrees, how
would that change the inclination?
Inclination would now be 180 degrees minus original inclination = 150
Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
8.
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Will the inclination change with increase in altitude?
No no no no.
Understanding Orbits
Earth System
Orbit plane
Orbit inclination
q
Ascending node
crossing the equator
r
Earth radius
h
Orbit altitude
9.
No
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Will the inclination change if the orbit were elliptical?
Understanding Orbits
Earth’s rotation around the sun
Equinox
Winter
Summer
Equinox
10. If there are no perturbations to the orbit, can you draw a terminator
orbit for each season?
Super bunch!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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Understanding Orbits
Earth’s rotation around the sun
Equinox
Winter
Summer
Equinox
11. Does the eclipse of the earth change with seasons?
Changes directions, but not shape or size.
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Understanding Orbits
All orbits go thru the
center of the earth
12. Why must all earth orbits rotate around the earth’s center of gravity?
Forces of gravity make it that way.
That is just the way it is. Nature is in control.
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Understanding Orbits
Earth’s rotation around the sun
Equinox
Winter
Summer
Equinox
13. Does the terminator or noon-midnight orbit differ for each season?
Both change if no perturbations.
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Understanding Orbits
Noon – Midnight
Orbit
Spacecraft enters
eclipse
Spacecraft
Leaves eclipse
14. Will the eclipse time of the spacecraft change with an increase in
altitude? Why?
Because the time in eclipse is almost constant, but orbit time will increases.
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Understanding Orbits
Noon – Midnight
Orbit
15. What would be the percentage of eclipse time if a spacecraft could
orbit at zero altitude?
SSE-120
Half the period or 50%.
Understanding Orbits
Calculating eclipse time for
a noon-midnight orbit
r
h
r+h
q
f
f
q
Solution
Q = cos-1 [r/(r+h)]
f = 90 - Q
Eclipse time = [(2* f)/360] * orbit time
16. What is the eclipse time for an altitude of 750 km for the noon-midnight
orbit? Make a Excel (spread sheet) chart for eclipse time from 300 km
to 800 km by steps of 50 km.
SSE-120
Understanding Orbits
Calculating eclipse time for
a noon-midnight orbit
r
h
r+h
q
f
f
q
Solution
Q = cos-1 [r/(r+h)]
f = 180 - Q
Eclipse time = [(2* f)/360] * orbit time
17. What would be the maximum eclipse time for a 750 km altitude orbit if
the inclination was 90 degrees? Do you need more information?
Need orbit orientation toward sun --- noon-midnight, terminator, or in
between?
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Understanding Orbits
Changing orbit inclination.
T
A
Change of eclipse time.
B
N-M
C
18. How does the eclipse vary with inclination?
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From a terminator orbit, decrease in inclination will increase eclipse time
after entering the eclipse.
Understanding Orbits
Earth’s rotation around the sun for a
GEO stationary spacecraft.
Equinox
Winter
Summer
Equinox
19. Does an eclipse occur with a GEO stationary spacecraft?
Yes, during equinoxes.
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Understanding Orbits
Earth’s rotation around the sun for a
GEO stationary spacecraft.
Equinox
Winter
Summer
Equinox
20. How often during the year will there be an eclipse of a GEO stationary
spacecraft?
Twice – each equinox.
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Understanding Orbits
GEO stationary spacecraft.
Start here Thursday for Menifee
Equinox
What is the max eclipse time?
21. How would you calculate the maximum time of the GEO eclipse?
Same way as for the LEO spacecraft. Only altitude of GEO is larger.
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Understanding Orbits
GEO stationary spacecraft.
Equinox
After Equinox
Before Equinox
http://www.satellite-calculations.com/Satellite/satellite_suneclipse_info.htm
22. How long would be the period that the GEO would continue to have
some eclipse twice a year?
SSE-120
Find when time between entrance to eclipse to exit from eclipse. (days)
Understanding Orbits
Done
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