Messier Marathon dark sky window and the galactic plane

Messier special time of the year

The Messier Marathon window respresents a special time of year for
north latitude observers. It is the monthly dark sky window when the
galactic north pole transits the local meridian in the early morning
hours. This marks the best alignment of the galactic plane with the
north latitude observer's local horizon.

In the celestial coordinate system, the Galactic North Pole is located
at J RA12h51m26s Dec+27d07.7m near 41 Coma Bernice. The corresponding
South Galactic Pole is a at J RA00h51m26s Dec-27d07.7m in the
constellation Sculptor near alpha Sculptor.

My observing point is near Salt Lake City, Utah, Long. 111d53m25s
(111.87) W, Lat. 40d45m39s (40.77) N. The following screen capture
made from Cartes de Ciel using my observing point illustrates how the
North Galactic Pole passes below my local zenith on February 2, 2005 at
4:00AM, or around midnight-2:00am during the monthly March dark sky
window:

http://members.csolutions.net/fisher...41N_4_00AM.jpg
(215k)

At the most favorable time of the year, the North Galactic Pole passes
about 13-14 degrees below my local zenith (41 deg N minus 27 deg N). As
a consequence of my northern observing point, a portion of the galactic
plane lies below my local southwest horizon even at its most favorable
presentation.

This favorable alignment occurs each night in the northern hemisphere
at those times of the year when line of right ascension RA12h51m26s (or
about RA 13h00m00s) transits your local meridian. The line of Right
Ascension 13h00m00s transits for northern observers between February
and May of each year. This line of RA transits in the early morning
hours (4:00AM) during February and transits around 9:00pm in June. Then
the advancing summer twilight obscures further transits.

Illustrations of the local horizon system at my observing point for
Latitude 41N for February 2, 2005 at 4:00am and for June 3, 2005 at
9:00PM can be found at:

http://members.csolutions.net/fisher...41N_4_00AM.jpg
(215k)

http://members.csolutions.net/fisher...41N_9_30PM.JPG
(206k)

The following animation, generated using Carte de Ciel, illustrates how
the galactic plane aligns with the local observer's horizon as the
North Galactic Pole transits.

WARNING: The following is a 1.5-1.7 megabyte animated GIF file.
http://members.csolutions.net/fisher..._Animation.gif

Note the orientation of Earth's position to the galactic plane in each
season as its orbits the Sun:

http://stars5.netfirms.com/galecl2.gif
(in http://stars5.netfirms.com/milkway.htm )

As the animation begins in November 2004, the observer's local south is
at the bottom of the screen. The observer's meridian runs north-south
through the screen.

In this November 2004 orientation, your field of view is relatively
empty of dust, gas and stars towards the South Galactic Pole near alpha
Sculptor near the bottom of the screen. (For northern latitude
observers, since the South Galactic Pole is near the horizon,
atmospheric extinction remains a problem.) Unobstructed by dust, gas
and stars in the galactic plane, the Sculptor Group of galaxies around
the South Galactic Pole can be observed. See -

http://www.pari.edu/sgra/Guides/Atla...outh_pole.html
http://www.seds.org/messier/xtra/ngc/sclgr.html

The animation runs in one week intervals through June 2005 and shows
the local observer's horizon system at around 10:00PM each night.

As the animation nears its conclusion in June 2005, the North Galactic
Pole, marked by a three degree circle in Coma Bernice, comes in from
the left and crosses the observer's zenith. At that point, the entire
circumference of the galactic plane is coincident with the local
observer's horizon.

During the March darksky window, this transit and alignment occurs in
the early morning hours.

At this time, the observer is looking "up" through the galactic plane.
In this orientation, your field of view is relatively empty of dust,
gas and stars towards the North Galactic Pole and the observer's
zenith. Unobstructed by dust, gas and stars in the galactic plane, the
Virgo Cluster of galaxies near the North Galactic Pole can be observed.
See -

http://seds.lpl.arizona.edu/messier/more/virgo.html

A general diagram of transit times by lines of right ascension can be
found in the 2004 or 2005 editions of the Royal Astronomical Society's
annual Observer's Handbook. (Most public libraries or planetariums
carry a copy of the Handbook.) Specific local sidereal times for your
observing point can be obtained from the U.S. Naval Observatory web
application at:

http://tycho.usno.navy.mil/sidereal.html

When the galactic plane aligns with the local observer's horizon, it is
a special time of the year. For me, it is one of the special benefits
of the Messier Marathon dark sky window.

- Canopus56