November Moon at Apogee – And the Ecliptic

click on graphic to see it larger   Our Moon reaches apogee, (furthest from Earth), for this orbit, on Thursday November 26th. For this apogee the 12-day old waxing gibbous Moon will be at a distance of 31.82 Earth diameters, 252,211 miles (405,894 km) from the Earth.

   On the date of the apogee the 12-day old waxing gibbous Moon will be about 12o to the east from the planet Mars. The Moon will also be about 11o west from the outer planet Uranus. Currently Uranus has an apparent magnitude of around 5.7 meaning that it could be seen with the unaided in dark enough skies, or with telescopes and even binoculars – as long as the Moon is not in that part of the sky.

   Along the Ecliptic
   You may notice the arrangement of the planets spread across the horizon as shown in the graphic. Many objects in our solar system orbit the Sun in a path that is somewhat parallel with the Earth’s orbit, the ecliptic. All of the orbits are tilted or inclined away from the ecliptic however the 8 classical planets all have orbits that are inclined less than 7o from the ecliptic. Dwarf planets, asteroids, comets, for example, have orbital inclinations greater than 7o.
   The ecliptic also defines the Sun’s apparent path against the background of stars throughout the year. The planets are also in motion as they orbit the Sun as the video below illustrates.
   Take a short cruise along the ecliptic with the Sun!

   Does our Moon actually go around the Earth as this graphic shows? From our perspective on the Earth the Moon appears to circle around the Earth. However, in reality, the Moon orbits the Sun together with the Earth*
   *Click here to read my 2006 Scope on the Sky column “The Real Shape of the Moon’s Orbit”. (PDF)

Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to go to bobs-spaces.

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Double or Triple Dating?

Click on graphic to see it full size.

Click on graphic to see it full size.

   Tuesday evening after local time for sunset there will be an opportunity to see several interesting pairs of celestial objects divided among three planets, our Moon, and several asteroids as this graphic shows. Jupiter is the single this evening, over the southwestern horizon near the twin stars of Pollux and Castor. Higher above the southern horizon is the constellation Leo the Lion with its distinctive backward question mark star pattern. At the bottom of the question mark is one of the celestial pairs, the star Regulus and the nearby asteroid 2 Pallas. Continuing eastward is the Dwarf Planet Ceres paired up with one of the larger asteroids, Vesta. Mars and the blue-white star Spica form the base of a triangle with the two asteroids as the point. Looking further eastward for the third celestial pair, and outshining everything else is the very near full Moon and a few degrees away the planet Saturn.

Click on graphic to see it full size.

Click on graphic to see it full size.

   As this graphic shows the asteroid 2 Pallas is close enough to the star Regulus in Leo so that both fit within the field of view of binoculars. Regulus shines at magnitude 1.4 while about 2 degrees away is the 8th magnitude asteroid Pallas.

Caution: Objects viewed with an optical aid are further than they appear.
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

Asteroids and Life

Jupiter-Size Planet Orbiting Outside Asteroid Belt

   Based on recent observations a team of Astronomers are suggesting that the presence of an asteroid belt with a Jupiter-sized planet in orbit just outside the asteroid belt could be an advantageous situation to life developing on planets between the asteroid belt and the star these objects orbit. The thinking is that asteroids are held in place by the gravitational influence of the Jupiter-size planet and were prevented from coming together forming larger bodies by repeated collisions that ultimately reduces the sizes as well as ejecting some of the objects inward toward the Sun and collisions with inner planets. These objects could have been responsible for bringing some water and other ices to the inner planets, as well as having an impact (literally) on any life forms.
   With our solar system as a model the team of Astronomers observed stars and found evidence that was consistent with their model and ideas – suggesting perhaps another method for searching for exo-solar systems, especially those similar to ours with an asteroid belt and Jupiter-size planet.
   Click here to read the press release.
   The Asteroid Belt:

The Main Asteroid Belt

   This area of the solar system, between the orbits of Mars and Jupiter, consists of rocky and possibly icy objects of varying sizes and shapes. The largest member is the 957 km (595 miles), spherically shaped 1 Ceres, now designated as a dwarf planet. This was also the first asteroid discovered, and initially considered to be a planet, as were the next three asteroids to be discovered: Pallas (524 km, 326 miles) , Juno (274 km, 170 miles), and Vesta (512 km, 318 miles). Of all the asteroids currently discovered and cataloged (150,000), only Ceres had enough mass during its formation to have the gravitational strength to become spherical. Typically, all other smaller asteroids are elongated or oblong in their shape, and some even appear to be composed of smaller pieces held closely together by their mutual gravitational fields.
   Asteroids, like the planets, orbit the Sun in the same direction. However, they are in orbits that are more elliptically shaped and more steeply inclined, averaging 20–30°above or below the plane of the ecliptic. Quite often the asteroid belt is depicted as appearing to be crowded (see above picture), however individual asteroids are spaced quite far apart.
   While the greater majority of asteroids are located within the main asteroid belt, there are two areas along the orbit of Jupiter where there are a collection of asteroids known as the Trojan asteroids. They are located approximately 60° ahead and behind Jupiter and orbit the Sun with Jupiter. Trojans have darker surfaces than main belt asteroids and are more difficult to detect, but it is thought that there may be as many Trojan asteroids as there are within the main belt.
   In the past, the asteroid belt probably contained enough material to form a small planet. However, the gravity from the largest planet, Jupiter, and its orbital period around the Sun, set up orbital resonances with many of the asteroids during the formation of the solar system, disrupting their orbits and preventing the formation of a planet. Many of these disrupted objects were pulled from the main asteroid belt and became meteoroids with their own orbits around the Sun, or were ejected from the solar system, or simply crashed into other planets or moons. Some of these we now know as NEO, Near Earth Objects, and these represent potential impacts with our planet if orbital circumstances are right.

Wikipedia links to 17 Asteroid Missions.

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

Is Earth A Dwarf Planet?

   The title is sort of misleading but after reading a press release from the NASA WISE (Wide-field Infrared Survey Explorer) mission I started thinking about the IAU official definitions for a planet that distinguishes a planet from a dwarf planet. In particular definition # 3. (see below) This thinking resulted from reading the press release which described success in determining the colors of the Trojan Asteroids that orbit at two specific points along Jupiter’s orbital path. I thought, “wait a minute”, if a planet had to have cleared its orbital path (# 3) then Jupiter with these Trojan Asteroids, and in fact Earth, Mars, and Neptune with their own Trojan Asteroids, may need to be considered as dwarf planets — by the definition.
   However the authors of the definitions took these Trojan asteroids into consideration and as the full definition explains there may be objects like the Trojan Asteroids that while orbiting the Sun along a planet’s orbital path, these objects are where they are as a result of gravitational interactions or orbital resonances with that planet, Jupiter for example. The bottom line is that there are no other objects along Jupiter’s orbital path other than those placed and held there by Jupiter’s gravitational field. And the same applies to Mars, Earth, and Neptune. Interestingly there are even moons having Trojan Satellites.
   Is Earth a Dwarf Planet? No, it is one of the 8 planets in our solar system.

   The Trojan Asteroids near Jupiter are interesting in many ways including that they are different from the asteroids we find in the main asteroid belt for example. The colors of these Jupiter Trojan asteroids are also different. Did you know that asteroids had color? Jupiter has two pair of Trojan asteroids – both at the same angle from Jupiter with one leading and the other trailing Jupiter.

  1. Orbits the Sun
  2. Has enough mass to have a spherical or round shape (at or near what is called hydrostatic equilibrium)
  3. Has cleared its neighborhood – objects along its orbital path.

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