Planets and Stars in Motion

   Over the next several days (evenings) both inner planets, Mercury and Venus, are moving along their respective orbits approaching each planet’s eastern elongation. From the animated graphic you can see that the stars in the background, like the two ‘Twin Stars’ Pollux and Castor, are also moving but toward the western horizon. This is a regular motion of the stars caused by the Earth’s own orbit, revolution, around the Sun. As the Earth revolves the stars appear to move westward – a real motion not to be confused with the apparent motion of stars toward the west as the Earth rotates.

   On the evening of June 25th Mercury will pass within about 5-6o from the star Pollux.

   
   
   
   
   
   
   
   
   

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June Perigee Moon

   Our Moon reaches perigee, (closest distance from Earth), for this orbit on Thursday June 14th. At that time the Moon will be at a distance of 28.18 Earth diameters (359,500 km or 223,383 miles) from the Earth.

   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*

   Thursday evening May 14th, shortly after sunset local time (8:26 CDT), look toward the western horizon for a conjunction between a thin 1.25-day young waxing crescent Moon and the innermost planet Mercury. The two will be about 2-3o apart but very low over the western horizon.

   *Click here to read my 2006 Scope on the Sky column “The Real Shape of the Moon’s Orbit”. (PDF)

   Read this very informative article about the Earth-Moon system and their orbital motions, written by Joe Hanson. “Do We Orbit the Moon?”

   
   
   

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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Young Crescent Moon in Conjunction with Mercury

   Thursday evening June 14th the thin 1.25-day young waxing crescent Moon will be about 2-3o from the innermost planet Mercury as the two are setting about an hour after sunset local time.
    As the graphic shows both will be low over the horizon before the sky is dark so this may be an interesting challenge to see either one or both. On the other hand further east, higher above the horizon, and very bright appearing is the other inner planet, Venus.

   
   
   

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


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Mercury Reaches Superior Conjunction

orbital-positions   Tuesday June 5th the innermost planet Mercury reaches superior conjunction. At superior conjunction Mercury will be on the opposite side of the Sun. The graphic to the right shows the planet positions relative to the Earth and Sun for both inner planets and outer planets.

   While at this superior conjunction Mercury will not be directly in line with the Earth and the Sun – on the ecliptic. Mercury has an orbital inclination of 7o with respect to the ecliptic. So like our Moon, Mercury during each complete orbit, will cross the plane of the ecliptic moving north (ascending node) and also moving south (descending node). For this superior conjunction Mercury will be a few degrees north of the ecliptic having crossed the ecliptic on June 1st with its ascending node..

   
   
   

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

Moon in Conjunction with Mercury and Uranus

   Sunday morning May 13th the 27-day old thin waning crescent Moon and the planets Mercury and Uranus will be in a grouping that will fit within a binocular field of view. All three rise about 1 hour before sunrise.
   This may also be an opportunity to see a crescent Moon that is within about 1 1/2 days from its new Moon phase.

   
   
   

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

Jupiter So Close, but Yet So Far

   Thursday 10 May the outer giant ringed planet Jupiter will be at perihelion, its closest to the sun for this orbit. At perihelion, which coincidentally is 1 day after it was at opposition, Jupiter will be approximately 4.4 AU (658,230,631 km; 409,005,551 miles) from the Sun. With Jupiter this close to its opposition the planet will be rising at around sunset local time and will be visible during the night, and setting at around sunrise local time. Joining Jupiter in the morning skies are the planets Saturn and Mars, as well as the waning crescent Moon.

   
   
   

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

Mercury at Western Elongation

19jan-mercury-east-elongation   On Sunday April 29th Mercury, the innermost planet, will reach its orbital position known as greatest western elongation at 27.0o. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows. Even though it sounds confusing at western elongation for either Mercury or Venus the inner planet will be to the right of the Sun as we view them, meaning that at western elongation an inner planet rises in the east before the Sun rises. And at eastern elongation with the inner planet on the left side of the Sun the inner planet follows the Sun across the sky setting after the Sun sets.
orbital-positions
   From our perspective the orbits of Mercury and Venus appear to move from one side of the Sun to the other – out to the left (east) from the Sun to eastern elongation, then reverse and move westward (inferior conjunction) between the Earth and the Sun to western elongation. From there the inner planet moves eastward going behind the Sun (superior conjunction) and eventually reappearing on the eastern side of the Sun for an eastern elongation. Repeat over and over – do not stop!


   Mercury is visible in the morning skies just before sunrise local time, as this graphic shows.

   
   
   
   
   

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