A Well Balanced Moon

Click on image to see full size

Click on image to see full size

   The first quarter Moon this evening lies nearly centered on the fulcrum for the constellation of Libra the Scales as the banner graphic at the top of the page shows. The image to the right shows a view toward the southwest about an hour after local sunset time. The planet Venus is low over the horizon and if you imagine an arc-shaped line curving upward toward the left, or east, along this arc is Spica, then Saturn, and then the first quarter Moon. The inset graphic shows the Moon within 1 degree of the third magnitude star Zubenelgenubi in Libra as would they would appear through 7×50 binoculars.
Click on image to see full size

Click on image to see full size

   With regard to the above written “arc-shaped line curving upward toward the left, or east” this arc-shaped imaginary line is in effect known as the ecliptic. The ecliptic in reality is the orbital path of the Earth around the Sun. However from our perspective we see the ecliptic as the apparent path the Sun follows against the stars in the background. It is the ecliptic that defines the constellations of the zodiac for example. Somewhere within a few degrees on either side of the ecliptic are where the 8 planets, Mercury – Neptune, are located as we view them from Earth. Our Moon also follows a monthly path that takes it above and below the ecliptic.
ecliptic.

   Click here to go to the Qué tal in the Current Skies web site for more observing information.

It’s Aries Time

Along the Ecliptic

Along the Ecliptic

   At 5 p.m. CDT (22 UT) on the 18th, today, the sun in its apparent eastward motion along the ecliptic crosses the boundary between the constellations Pisces the Fishes and Aries the Ram as it enters the area of the sky containing the stars of Aries. This graphic shows the real position of the Sun on this date.
   The banner graphic at the top of this page shows a larger view of the area including the point, Vernal Equinox, where historically it was used to mark the starting point for the change of seasons from northern hemisphere winter to spring. This celestial position is referred to as the ‘First Point of Aries‘ even though the Sun is no longer at that position at the start of spring around March 20th. The shift of approximately one month is a result of a long term motion known as precession, or precession of the axis. precession circlePrecession is a wobble like motion the Earth has caused by the gravitational pull of the Sun and to a lesser extent, the Moon. The Earth leans at an angle of 23.5 degrees and over time, about 26,000 years the Earth wobbles and the poles of the Earth trace out a circle where any star on or near the precession circle will be the pole star.
   Another effect of precession, as the Earth wobbles, is that the celestial coordinate system, which is based on the Earth’s geographical coordinate system (latitude and longitude) moves or shifts toward the east with respect to the stars and constellations. The stars in the background stay fixed, more or less, in their location, while the celestial coordinates move as the Earth is precessing. The result is that several centuries ago the sun was in Aries at the start of spring, now the Sun is further west across Aries and has nearly precessed into Pisces.

      Click here to go to the Qué tal in the Current Skies web site for more observing information.