
Astronomical Parallax 3D
General Description
This simulation illustrates the phenomenon of annual stellar parallax in astronomy.
Parallax is the apparent displacement of an object relative to the
background that is caused by the motion of the observer (rather than
the motion of the object itself, or of the background). This simulation
illustrates the parallax of an object in space that results from the Earth's annual
orbital motion.
The Space View window shows the Earth (blue point) orbiting the Sun
(organe point). The white point represents a stationary star. The open white circle shows the location of
the object on the celestial sphere as seen from the Sun. The colored
(initially magenta)
point shows the location of the star on the celestial sphere as seen by an observer on the
orbiting Earth. Various options allow the user to display the line of
sight from Earth through the star, the line from the Sun through the
star, cardinal direction arrows, the trace of the star's apparent
motion, the trace of Earth's orbit, and the planes and axes of the
celestial equator and ecliptic. There are also options to display
plots of the apparent right ascension and declination of the star as
functions of time. Controls allow the user to adjust the
distance to the star as well as its celestial coordinates (as seen
from the Sun). 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. A menu allows the user to select a particular
day (equinox or solstice) of the year.
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 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 distances to the "star" are
vastly smaller than the distance to any real star (at the distance to
the nearest real star the annual parallax would be imperceptible in
this simulation). The size of the Earth, sun, and star are
exaggerated so as to make them visible on the scale of the simulation.
Space View Frame
This frame shows a view of the Earth orbiting the sun, the star to be
observed, and the "true" and apparent locations of the star 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-Star Line: show line from Earth through star.
- Show Sun-Star Line: show line from Sun through star.
- 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.
- Orange point: Sun.
- White point: star.
- White open circle: apparent location of star as seen
from Sun.
- Colored point: apparent location of star as seen from
Earth.
- Colored line: trace of star's apparent motion.
- Colored arrow: line of sight from Earth through star.
- Orange line: line of sight from Sun through star.
- 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.
- Distance: distance to the star (in Astronomical Units).
- 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: location of star on sky as seen from
Sun.
- Colored disk: location of star on sky as seen from
Earth.
- 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)