Changes in the apparent position of the galactic plane and celestial sphere through figure and animations

fisherka@csolutions.net Rev. 7/17/2006

• Understanding what the plane of the Milky Way Galaxy looks like at various times of the year

A beginning point for understanding the Milky Way Galaxy is to know what parts of the galaxy are seen throughout the year as the Earth completes one orbit of the Sun. This understanding requires gaining an appreciation of the tilt of the Earth's orbit (the plane of the ecliptic of the Solar System) with respect to the galactic plane at the equinoxes and the solstices. The tilt of the ecliptic plane with respect to the galactic plane is extreme - about 62 1/2 degrees.

Another consequence of the Earth's orbit relative to the galactic plane is that amateurs see a limited part of the Milky Way during each major season. These effects can be seen in Figures 2-9, below. The galactic plane is defined visually as a swath of the sky about 10 degrees above and below the galactic mean plane. Alex Mellinger's classic photo, The Milky Way Galaxy illustrates this 10 degree swath visually. External content link to Alex Mellinger's Virtual Reality Milky Way Panorama website. The galactic plane runs through the following constellations that contain bright stars - Sag Cep Gem Sct Cas CMa Aql Per Vel Cyg Aur Car Cen

Figure 1 - Ecliptic Plane and Galactic Plane illustrated - 3D Snapshot

A result of the high inclination of the Earth's orbital plane to the galactic plane is that the Earth's view varies widely throughout the year.

• At the vernal or spring equinox, the Earth is at its highest northern point relative to the galatic plane. The Milky Way's galactic plane is nearly coincident with the observer's horizon. Figure 2. An observer at 40 degrees North latitude looks "up" and "out" the top of the Milky Way's plane through the relatively star-free region of the Coma Bernices.
• Three months later at the summer solstice, the Earth has moved counterclockwise one-quarter of its orbit and the observer at 40 degrees North latitude looks "sideways" towards the Milky Way's galactic core and the rich star and gas regions of the Sagittarius Arm and the constellation Sagittarius, e.g. - the Lagoon Nebula, Messier 8. At midnight on the summer solstice, the plane of the Milky Way appears to run north and south relative to the observer's horizon. Figure 3.
• At the autumnal equinox, the 40 North latitude observer now looks "down" and "out" the Milky Way's plane towards the south galactic pole. Figure 4.
• At midnight on the winter solstice, the observer is again looking "sideways," through rich star fields and gas nebula in the galactic plane to the next galactic arm out - the Perseus Arm. The Orion Nebula, M42, is located in this direction, as is the "unwinding" anti-spinward direction of our local Orion Arm. Figure 5.

The following are 360 degree altitude-azimuth charts of the night sky at 12 midnight and 9pm at the equinoxes and the solstices, generated using Cartes du Ciel:

Figure 2 - Milky Way at Vernal/Spring Equinox at 12am from 40 deg N lat	Figure 3 - Milky Way at Summer Solstice at 12am from 40 deg N lat	Figure 4 - Milky Way at Autumnal Equinox at 12am from 40 deg N lat	Figure 5 - Milky Way at Winter Solstice at 12am from 40 deg N lat
Figure 6 - Milky Way at Vernal/Spring Equinox at 9pm from 40 deg N	Figure 7 - Milky Way at Summer Solstice at 9pm from 40 deg N lat	Figure 8 - Milky Way at Autumnal Equinox at 9pm from 40 deg N	Figure 9 - Milky Way at Winter Solstice at 9pm from 40 deg N lat

The orientation of the Earth in its orbit relative to the galactic plane is best understood by taking mental snapshots of the night sky at midnight at the equinoxes and the solstices. However, most amateurs are used to seeing the night sky at 9pm local time. The Earth's daily rotation also effects the apparent orientation of the Milky Way to the observer. In the foregoing figures, the night sky as seen at midnight and 9pm local time are illustrated. Comparing Figures 2-7 and 5-6, there is an evident pattern. The sky at midnight at the spring equinox (Figure 2) looks nearly identical to the sky at 9pm on the summer solstice (Figure 7). Similar pairings are seen for the other equinoxes and solstices. Figures 3 and 8, and 4 and 9.

The solar apex - the apparent direction of travel of the Earth within the Milky Way - in the constellation Hercules appears in Figure 3, Summer Solstice at 12AM, and in Figure 8, Autumnal Equinox at 9PM, above. In the spring at midnight, amateurs can look "up" and "out" the "top" of the Milky Way's plane and have a relatively unimpeded view of distant galaxies. Figure 2. At the summer solstice at midnight, rich star fields that are dense with gas clouds and open clusters in the galatic plane towards the galactic core (galactic longitude 0 degrees) are visible. The "spinward" portion of our local Orion-Cygnus galactic arm is visible through 90 degrees galactic longitude. Figure 3.

Figure 10 - Proper motion of nearby stars during the last 10,000 years and the solar apex - from Cartes du Ciel

At the autumnal equinox, the amateur can see portions of the Perseus Arm stretching east to west along the local zenith from Cygnus (90 degrees galactic longitude) to Gemini (~180 degrees galactic longitude). Figure 4. But the observer can also see "down" and "out" the Milky Way's plane towards the south galactic pole - into a relatively star free region and towards distant galaxies in the constellation Fornax. Figure 4. At the winter solstice, the amateur again sees the rich star fields full of open clusters and nebula, like M42 in Orion, all contained in the Milky Way's galactic plane. Figure 5. Near the winter solstice, the northern observer sees from the galactic anti-center near Gemini towards galactic longitude 270 degrees in the constellation Vela. This portion of the sky, near Canis Major, also contains the "unwinding" or "antispinward" portion of our local Orion Cygnus Arm.

The change in apparent orientation of the galactic plane between the equinoxes and solstices is illustrated in the following animations.

• From 41 North Latitude between November and the following spring equinox: The following animation, generated using Carte de Ciel, illustrates how the galactic plane nearly but not exactly aligns with the local observer's horizon as the North Galactic Pole transits near the spring equinox.


Figure 11 - From 41 deg N lat between November and March (1.2 Mb)

In the beginning November orientation of the animation, the bottom one-half of your field of view is relatively empty of dust, gas and stars towards the South Galactic Pole near alpha Sculptor near the bottom or south horizon of the screen. Galaxies in this unobscured region around the South Galactic Pole can be viewed. The Milky Way appears as an east-west running band. This is the Perseus Arm - the next arm out in our spiral galaxy. Amateurs search for intra-galactic objects in that arm of the Milky Way, such as planetary nebula, open clusters, and darka and bright nebula.

As the animation continues to winter, the Milky Way runs north-south across the screen and amateur astronomers focus on intra-galactic objects.

As the animation continues through spring and through June, the Milky Way's galactic plane starts to align with the observer's horizon as the constellation Leo marches in from due East. This leaves the observer's zenith unimpeded by gas, dust and stars in the Milky Way. Amateur astronomers then hunt out distant galaxies around the North Galactic Pole.

• From 27 South Latitude between November and the following spring equinox: The following animation, generated using Carte de Ciel, illustrates how the galactic plane aligns with the local observer's horizon near the spring equinox. Since the galactic north pole is at 27 deg North declination, from this location the North Galactic Pole passes directly through the observer's zenith. At 41 North latitude, Figure 11, the North Galactic Pole is offset from the zenith and the galactic plane never exactly aligns with the observer's horizon. In Figure 12 at 27 South latitude, the galactic plane almost exactly aligns with the local observer's horizon. The same occurs at 27 degs North latitude, but six months later.


Figure 12 - From 27 deg S lat between November and March (1.2 Mb)

Locations in the United States that have favorable congruent alignments between the galactic plane and the local observer's horizon include:

     Falfurrias, Texas, at 
     Lat:  27.220899  27:13:15.236N 
     Lon:  -098.145031  98:08:42.112W 

about 80 km southwest of 

     Corpus Christi, Texas at 
     Lat:  27.794079  27:47:38.684N 
     Lon:  -097.400253  97:24:0.911W 

and 
     Osprey, Florida at 
     Lat:  27.197776  27:11:51.994N 
     Lon:  -082.492610  82:29:33.396W 

about 40km south of 

     Saint Petersburg, Florida at 
     Lat:  27.772802  27:46:22.087N 
     Lon:  -082.638327  82:38:17.977W 

Suitable maps can be generated via Mapquest.com at http://www.mapquest.com/maps/latlong.adp.

• Effect of Precession on the Appearance of the Celestial Sphere

In order to better understand precession, as a 50 arcsec per year "wobbling" of the Earth's central axis, two animated gif simulations illustrating what precession looks like over a long period of time were generated using Cartes du Ciel.

• A simulation of precession as seen by an observer at the North Pole looking at the Celestial North Pole at the zenith from December 31, 1956, for 122 years (1.9 Mb)
• A simulation of precession as seen by an observer at Salt Lake City, Utah (41N 111W) of the celestial equator at the First Point in Aries on Sept. 21, 1956 for the next 89 years (1.5 Mb)

The effects of precession are more pronounced at the equator than near the North Celestial Pole. Precession will be difficult to see in the first animation, but easy in the second.

In the first simulation at a North Pole observing point, the observer looks up at the Celestial North Pole on December 31, 1956. Polaris is in the upper right-hand corner of the field of view (FOV). The field grids are the observer's local horizon, which at this location is congruent with the equatorial system. A bull's eye of 1', 10', 20' and 30' arcmins marks the location of the Celestial North Pole on December 31, 1956, and provides a frame of reference of next 122 years of simulation. The FOV is approximately 1.5° by 2.5°. Stars are plotted to a deep magnitude 18 in order to provide visual frame of reference. As the simulation proceeds there is slight annual shift in the TFOV orientation due to my inability to find within the planetarium software a precise setting that would maintain a fixed field of view. The animation proceeds in one calendar year increments for 122 years. General precession, which moves at approx. 50" per year, is illustrated by watching how the central 1' arcmin bulls eye moves slightly away from the current epoch's (year's) Celestial North Pole.

In the second simulation at a 41N observing point, the observer is looking at the spring equinox or the First Point in Aries, which on the vernal equinox is high in the night sky at the intersection of the celestial equator and the ecliptic. The bulls eye marks the fundamental point of the equatorial system (the intersection of the celestial equator and the ecliptic at RA 0h0m0s Dec 0d0m0s) on September 21, 1956 at 9:00pm. The FOV is approximately 1.5° by 2.5°. Stars are plotted to a deep magnitude 18 in order to provide visual frame of reference. As the simulation proceeds there is slight annual shift in the TFOV orientation due to my inability to find within the planetarium software a precise setting that would maintain a fixed field of view. The simulation progresses in one calendar year plus 56 second increments for the next 89 years. General precession, which moves at approx. 50" per year, is illustrated by watching how the central 1' arcmin bull's eye moves slightly away from the current year's spring equinox. The three brightest stars in the bull's eye are HD224958 (v9.1), HD224876 (v9.2), and HD224934 (v9.4).

These simulations do not address nutation, which is an additional periodic movement within an approx. 40" x 80" arcsecond ellipsis over 18 years. Nutation is caused by the Moon tugging on the Earth. That ellipsis is within the central 1' arcmin of the bull's eye. There was not setting in Ciel fine enough to capture the nutation movement.