May Moon at Apogee

   Our Moon reaches apogee, (greatest distance from Earth), for this orbit, on Monday 18th. At that time the 26.5-day old waning crescent Moon will be at a distance of 31.88 Earth diameters 252,028 miles (405,600 km) from the Earth.

   On the day of the apogee the thin waning crescent Moon rises about 1-2 hours before sunrise local time. Looking carefully with binoculars you may be able to see nearby 4th magnitude star 20 Ceti, one of the many stars that are part of the constellation Cetus the Whale.

   

   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)

   
   
   
   
   
   
   
   
   

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Mars – Jupiter at Heliocentric Conjunction

   Saturday May 16th the outer planets Mars and Jupiter will reach a point in their respective orbit that has them at nearly the same heliocentric coordinates. This a system of tracking the planets as they make their 360o orbit around the Sun. Each Sun orbiting object’s orbital position is measured using degrees, minutes, and seconds of heliocentric longitude. Each object orbits the Sun at a daily rate determined by dividing 360o by the number of days an object takes to complete one orbit around the Sun.
   As the above graphic shows the two planets are arranged in a straight line out from the Sun. The Earth is not part of the line-up. This is a heliocentric view of their orbital positions with the Sun at the Center. A heliocntric longitude based view is not the same as a view from the surface of the Earth where there is a distinctly different view of the two planets.


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Say it Ain’t So!


   According to the web site Physics-Astronomy on May 16th the crescent Moon will be in a conjunction with the inner planet Venus and an outer planet, Jupiter, in a formation looking like a smiley face.

   This type of conjunction has happened in the past including one in 2 B.C. that has been suggested to be the ‘Star of Bethlehem’. More recently this triple conjunction did happen in December 2008, according to the article.
   Click here to see a list of future planetary conjunctions.

   Looking down from above the solar system on May 16th showing the orbital positions of Venus, the Earth, and Jupiter. From this graphic it shows that from the Earth you would have to look toward different directions to see either planet.

   No this will not happen.
   Here are reasons why this article about the conjunction is “fake news”.

   Venus is visible in the evening skies at sunset over the western horizon and sets around 9-10 pm local time. (Rises: 7:23 am – Sets: 10:24 pm)
   Jupiter is visible in the morning skies over the southern horizon and sets during the afternoon local time. (Rises: 12:45 am – Sets: 10:47 am)
   The article does not indicate which crescent Moon. Is this a waxing crescent Moon in the evening or a waning crescent Moon in the morning?
   The Moon is in fact a 22.8-day old waning crescent Moon in the morning skies. However the Moon is closer to Neptune and Dwarf Planet Ceres than it is to either Venus or Jupiter.



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Moon Mars Conjunction x2

   Thursday morning May 14th the last quarter Moon will be about 8-9o to the west from the ‘Red Planet’ Mars. The following morning, Friday May 15th the waning crescent Moon will be about 6-7o from Mars but this time on the east side.



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Two More Go Retro!

   Retrograde motion is an apparent motion of any Sun-orbiting object as viewed from one object to another object further from the Sun than the one the viewing is made from. Since we are based on the Earth our views of the planets and other objects orbiting the Sun come from that perspective. As I posted the other day, outer planets. for example, orbit the Sun more slowly than the Earth. So there are times when, as the Earth passes an outer planet, the outer planet appears to slow down and then move backward, to the west. After a period of time the planet resumes its regular eastward, or prograde, motion.
   The retrograde motion of an outer planet is easy to understand and even visualize, however the two inner planets also undergo retrograde motion. Half of each their respective orbit is eastward, prograde, but when they reach the opposite side of the Sun their orbit carries the inner planet around the Sun through inferior conjunction (between the Earth and Sun) toward the west, retrograde, for the other half of the orbit.
   For the record each ‘side’ of the orbit is known as an elongation. So at western elongation the inner planet is on the west side of the Sun and rises before the Sun rises – a morning planet. Half an orbit later the inner planet is at eastern elongation and rises and sets after the Sun – an evening planet.
   Venus is currently very prominent as an evening planet over the western horizon at sunset. The planet Mercury has recently passed through superior conjunction is gradually moving into the evening skies. Mercury is moving in prograde motion while Venus is moving in retrograde motion.


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Moon-Jupiter-Saturn Conjunction

   Tuesday morning May 12th, before sunrise, the 19-day old waning gibbous Moon will be within a few degrees from the outer planets Jupiter and Saturn. All three will fit within the field of view of binoculars.

   
   
   

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Saturn Goes Retro!

   All Sun orbiting objects further from the Sun than the Earth will have orbital speeds slower than the Earth’s orbital speed. From our perspective on Earth orbital directions are toward the east.
   There will come a time when the faster moving Earth will overtake a slower mover and pass by that object much like a car passes a slower moving car on the highway.
   During the time that Earth is passing by an outer object there is an appearance that the outer object slows down and then orbits in the reverse direction, toward the west.
   This is known as retrograde motion. After a few weeks to a few months, depending on the outer object, the apparent westward motion slows to a stop and then the outer object resumes its regular motion toward the east known as prograde motion.
   Such is the situation for outer ringed planet Saturn. It begins retrograde motion this month and will resume its regular eastward, prograde, motion during September.

   Take a short tour of Saturn and some of its moons in this video clip from the longer Orbits video. These were part of a live performance by the group Dark Matter.

   
   
   

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