Mercury at Superior Conjunction

orbital-positions   Tuesday May 21st 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 had just crossed the ecliptic moving north.
   

   
   
   

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Waning Crescent Moon – Venus Conjunction

   Wednesday and Thursday mornings, May 1st and 2nd the 26 to 27-day old thin waning crescent Moon will pass by the inner planet Venus coming the closest on the 27th when the two will be separated by about 3-4o. A day later, Friday, the very thin waning crescent Moon will be near Mercury and the outer planet Uranus, but the Sun will be less than 45 minutes behind them – meaning the sky may be too bright to see the Moon or Mercury.
   Further to the west the outer planets Saturn and Jupiter are visible in the area around Sagittarius and Scorpius. Even a bit further west is the closest Dwarf Planet to the Earth, former asteroid Ceres.


   
   
   

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April Moon at 2nd Apogee

   Our Moon reaches apogee, (greatest distance from Earth), on Sunday April 28th. At that time the Moon will more or less be at a distance of 31.85 Earth diameters 251,392 miles (404,577 km) from the Earth.

Does our Moon actually go around the Earth? 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*

   On the day of the apogee the 24-day old waning crescent Moon will be over the southeastern horizon 1-2 hours before sunrise local time.

*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 return to bobs-spaces.


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A Tale of Two Phases

   Over the next couple of weeks the two inner planets, Mercury and Venus, will be above the eastern horizon in the hour or two before the Sun rises. Given its much brighter apparent magnitude (-3.92) Venus will be more noticeable. However Mercury (0.03) is only a few degrees away, down to the left as they both rise. This graphic shows the two planets (sizes greatly enlarged for the graphic) and their respective orbital paths. Both planets are moving eastward toward the Sun and will eventually catch up with and move to the opposite side of the Sun, behind the Sun as we view from Earth to their respective superior conjunction.

   As these two planet orbit toward the Sun they pass through phase changes much like our Moon. Currently both planets are waxing gibbous but as they move into superior conjunction they will be at full phase. But unlike our Moon at full phase the inner planets are not visible at their respective full phase. When they do reappear on the east side of the Sun they will be again at gibbous phase, and will be following the Sun across the sky, setting after the Sun as an ‘evening planet’.

   
   

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Moon – Jupiter Conjunction

   Tuesday morning April 23rd the 18-day old waning Gibbous Moon will be in a close conjunction with Jupiter as the two are separated by about 1o. The two should make for an interesting view as they both will very easily fit within the field of view of 7×50 binoculars.

   
   
   

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The Moon and a Dwarf Planet

   Monday morning April 22nd, before the Sun rises, look toward the south-southwest for the 17-day old waning gibbous Moon. While the Moon is obviously easy to see at a -12.60 apparent magnitude, the nearby, (2-3o), dwarf planet Ceres with an apparent magnitude of 6.90 is outshined by the Moon and is not visible.
   As this graphic shows all of the naked-eye visible planets except Mars are arranged from west to east above the horizon. While not naked eye visible Neptune, with an apparent magnitude of 7.94, is also shown. This arrangement of planets then offers an opportunity to visualize the plane of the ecliptic, the Earth’s orbit extended onto the sky. The plane of the ecliptic is one of the primary frames of reference for our solar system, and one of the things the other 7 planets have in common is that their respective orbits are all within about 7o from the plane of the ecliptic. Even our Moon stays within about 6o from the ecliptic.
   
   
   

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An Inner Planet Get Together

   Tuesday morning April 16th before the Sun rises watch for the two inner planets Mercury and Venus to be rising together and separated by about 3-4o. Both planets will easily fit within the field of view of 7×50 binoculars.
   Compare the apparent magnitude of Venus, -3.93 with the 0.22 apparent magnitude of Mercury.

   This is the closest the two will come to each other but Mercury and Venus will be visible above the horizon for the rest of the month and into May. Both planets are moving eastward along their respective orbital paths. This animated graphic is set to 1-day intervals from April 16th to April 30th.

   
   
   

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