
Stellar Aberration 3D
General Description
This simulation illustrates the phenomenon known as the aberration of
starlight, first reported by James Bradley in 1729. Aberration occurs because light has a finite speed, and
thus light from a star takes a finite amount of time to travel through
the tube of a telescope. During this time, the telescope moves as a
result of Earth's rotational and orbital motions (in this case, the orbital motion
is more important because it is faster). Therefore, if the telescope
is pointed directly at the star the starlight will hit the sides of
the tube before reaching the eyepiece. To see the star the telescope
must be pointed forward (ie in the direction of Earth's motion) very
slightly. As a result the apparent location of the star on the sky is different
from its true location.
The Space View window shows the Earth (blue point) orbiting the Sun
(orange point). The open white circle shows the location of
the star on the celestial sphere. The colored (initially magenta)
point shows the apparent location of the star on the celestial sphere,
which is shifted from the true location because of the aberration
effect produced by Earth's orbital motion. Options allow the user to
display the Earth's velocity vector and its orbital path, the vector
sum that produces the aberration effect (the velocity of light from
the star minus the velocity of Earth gives the apparent velocity of
light from the star), the equatorial and ecliptic planes and axes,
direction arrows, and a trace of the star's apparent motion due to aberration. There are also options to display
plots of the apparent right ascension and declination of the star as
functions of time. A menu allows the user to select a particular
day (equinox or solstice) of the year. Controls allow the user to
adjust the time of year, speed of Earth, and coordinates of the star. The color of the apparent star (and its traces, etc)
can be changed so that multiple traces can be shown at once, each in a
different color.
The Sky View window shows the apparent location of the star on the
sky as seen by an Earth observer. The "true location" (the location
as seen from the Sun) is shown as an open white circle, while the
apparent location (effected by aberration) is shown as a colored
disk.
Note that some features have been simplified or exaggerated. The
Earth's orbit is treated as a circle. The size of Earth, Sun, and
Earth's orbit are greatly exaggerated compared to the distance to the
star. The speed controls allow the
user to set Earth's orbital speed to an appreciable fraction of light
speed, which is not realistic. Finally, the simulation illustrates the
"classical" aberration effect, not the (more correct) relativistic aberration.
Space View Frame
This frame shows a view of the Earth orbiting the sun, the "true"
and apparent locations of
the star to be
observed on the Celestial Sphere, as seen from outside the Sphere.
- Viewing Options Menu
- Show Equatorial Plane: show disk representing the plane of
Earth's equator (but passing through the Sun).
- Show Ecliptic Plane: show disk representing the plane of
Earth's orbit.
- Show Celestial Axis: show line representing the axis of
the celestial sphere (parallel to Earth's rotational axis
but passing through the Sun).
- Show Ecliptic Axis: show line representing the axis of
Earth's orbit.
- Show Directions: show cardinal direction arrows on the
celestial sphere near the object's apparent location (in
both views). Note that in the Sky View window you are
looking out from the interior of the celestial sphere, so
the E-W directions are reversed.
- Show Earth's Velocity: show the velocity vector of Earth.
- Show Vector Sum: show the vectors for the velocity of
light from the star heading toward Earth, the velocity of
Earth, and the apparent velocity of the starlight. Note
that the velocity of the starlight minus Earth's velocity
gives the apparent velocity of the starlight (classically).
- Show Celestial Sphere: show the grid representing the
celestial sphere.
- Show Trace: show trace of the star's apparent motion.
- Show Earth Orbit: show trace of the Earth's motion.
- Show Plot of Apparent RA vs. Time: displays a plot of
the apparent right ascension of the star as a function of
time (with time given in years since the vernal equinox).
- Show Plot of Apparent Dec vs. Time: displays a plot of
the apparent declination of the star as a function of
time (with time given in years since the vernal equinox).
- Select Day Menu
- Vernal (Spring) Equinox: set day to vernal equinox
(roughly March 21).
- Summer Solstice: set day to summer solstice
(roughly June 21).
- Autumnal (Fall) Equinox: set day to autumnal equinox
(roughly September 21).
- Winter Solstice: set day to winter solstice
(roughly December 21).
- Visual Elements
- Blue point: Earth.
- Blue line: trace of Earth's motion.
- Cyan arrow: vector for Earth's velocity.
- Orange point: Sun.
- White open circle: star.
- Colored point: apparent location of star due to aberration.
- Colored line: trace of star's apparent motion.
- Colored arrow: vector for apparent velocity of starlight.
- White arrow: vector for velocity of starlight heading
toward Earth.
- Green arrows: direction arrows.
- Cyan disk/line: equatorial plane/axis.
- Red disk/line: ecliptic plane/axis.
- Controls
- Buttons
- Play/Pause: start and stop the simulation.
- Reset: reset the simulation to its initial state.
- Clear traces: clear all traces in Space and Sky Views.
- Change Color: changes the color of the apparent star
and its traces, etc. The new color is selected at random.
- Sliders
- Time: time of year (in years) beginning at the vernal
equinox.
- Speed: speed of Earth's orbital motion divided by speed
of light.
- RA: right ascension of the star as seen from the Sun.
- Dec: declination of the star as seen from the Sun.
Sky View Frame
This frame shows an observer's view of the "true" and apparent
locations of the star as seen from Earth.
- Visual Elements
- White open circle: true location of star on sky.
- Colored disk: apparent location of star on sky due to aberration.
- Colored line: trace of the star's apparent motion.
- Green arrows: cardinal direction arrows (recall that
this view is looking up, out of the celestial sphere).
RA Plot Frame
This frame shows a plot of the apparent right ascension of the star as
a function of time (with time given in years since the vernal equinox).
- Visual Elements
- Colored disk: point denoting the apparent right
ascension of the star at the current simulation time.
- Colored line: trace of the apparent right ascension of
the star as a function of time.
Dec Plot Frame
This frame shows a plot of the apparent declination of the star as
a function of time (with time given in years since the vernal equinox).
- Visual Elements
- Colored disk: point denoting the apparent declination of the star at the current simulation time.
- Colored line: trace of the apparent declination of
the star as a function of time.
Todd K. Timberlake (ttimberlake@berry.edu)