Author Archives: Bob Riddle

About Bob Riddle

Currently: Retired Teacher (May 2017) but not yet finished learning and teaching. Column Editor, Scope on the Skies - Science Scope Magazine; NASA/JPL Solar System Educator Program - Solar System Ambassador Program, and Solar System Ambassadors Program.

Pluto at 2019 Opposition

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   Monday July 15th the Dwarf Planet Pluto will be at opposition. All outer planets and other solar system objects that orbit the Sun beyond the orbit of the Earth have opposition. At that orbital position the Earth is between the Sun and the outer solar system object, much like the Sun-Earth-Moon arrangement for a full Moon. At opposition the outer solar system object rises at the local time for sunset and sets at the local time for sunrise – again just like the full Moon.

   Where is Pluto and is Pluto visible to the naked eye? Pluto currently is a few degrees east from the handle of the teapot-shaped asterism for Sagittarius the Archer, and a few degrees east from Saturn. Pluto is visible, but with an apparent magnitude of 14.8 Pluto would only be visible with large aperture telescopes and with time-exposure pictures.

On July 14th 2015 the New Horizons spacecraft flew past the dwarf planet Pluto. Click here to learn more about the New Horizons mission and take part in the mission with some of the interactives created by NASA.

   
   
   
   
   
   
   
   

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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Moon – Saturn Conjunction

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   Mid-evening Monday July 15th as the nearly full Moon rises it will be in a close conjunction with the outer ringed planet Saturn. The two will be separated by about 1-2o and both will easily fit within the field of view of binoculars, or a wide field telescope eyepiece.

   
   
   

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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Moon – Jupiter Conjunction

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   Saturday evening July 13th the 11-day old waxing gibbous Moon will be about 2o from the outer ringed planet Jupiter as the two rise in the east around sunset local time. Both will nicely fit within the field of view of binoculars.
   Using the binocular field of view (FoV) as a ‘ruler’ shift the binoculars about 1 FoV to the west, right, putting Jupiter on the left edge of the FoV and the reddish star Antares will be at the right edge.

   
   
   

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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Saturn at Opposition – 2019

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   Tuesday July 9th the outer planet Saturn reaches its orbital position known as opposition. This is a position which has the faster moving Earth passing Saturn and at opposition is centered between the outer planet and the Sun. Picture the arrangement with the Moon at full phase; Sun – Earth – Moon, and that is similar to the arrangement for Saturn at opposition.
   When an outer planet, like Saturn, reaches opposition that planet rises around local time for sunset and is visible all night.
   The 7.5-day old waning gibbous moon will be west from Saturn and a few degrees above the bluish-white star Spica in Virgo the Harvest Maiden – both over the southwestern horizon. Look for the planet Jupiter and the reddish star Antares, the ‘heart’ of Scorpius the Scorpion to be just to the west from Saturn.

   
   
   
   
   

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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Moon Along the ecliptic

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   As the Earth and Moon orbit the Sun together the Moon follows an orbital path that takes it along the plane of the ecliptic (Earth’s orbit), sometimes above, and sometimes below. At least twice each orbit or during during the calendar period for that orbit the Moon will cross the ecliptic either as an ascending node or a descending node.
   As this short video shows the Moon will follow a path along the ecliptic and as it does so it will pass some of the brighter stars and planets that are arranged on or near the ecliptic.
   You may also notice a steady shift of the sky toward the west. This is the effect of the Earth in motion, revolving, around the Sun. Since the Earth covers the 360o orbit in approximately 365 days the Earth moves almost 1o each day, and the sky in turn has the noticeable westward shift of the same amount.

   
   
   

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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July Perigee Moon and A Conjunction with Regulus

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   Our Moon reaches perigee, (closest distance to Earth), for this orbit, on Friday July 5th. At that time the Moon will be at a distance of 28.51 Earth diameters, 226,011 miles (363,729 km) from the Earth.

   The 4.50-day old waxing crescent Moon is over the southwestern horizon at sunset local time and sets around midnight. About 1o east from the Moon is the ‘Heart of the Lion’, the star Regulus in Leo the Lion. The two will easily fit within the field of view of binoculars or a low power telescope eyepiece.
   Appearing lower above the western horizon are the planets Mars and Mercury.

   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)

   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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Earth at Aphelion – 2019

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Earth at Perihelion   Thursday July 4th, as the Earth continues its annual trek around the Sun, the Earth reaches a point in its orbit that is called aphelion. Aphelion is the greatest distance that separates the Earth from the Sun, and we are the furthest from the Sun for the year at this point in the orbit. So, at 23 UT (6 pm CDT) on Thursday July 4th the Earth is 1.01670 AU (94,508,169 miles; 152,096,155 km) from the Sun.
   Approximately one-half year or one-half revolution earlier, on January 3rd, the Earth was at perihelion, its minimum distance from the Sun for this year (0.9833 AU (91,403,445 miles; 147,099,586 km). This difference, about 3%, in distances is due to the shape of the Earth’s orbit being elliptical rather than circular. However the Earth has a mildly elliptically shaped orbit that is closer to being slightly out-of-round than the incorrect, very elliptical orbit that is often shown – like the illustration used here.
sun2014-ani   In Astronomy the shape of a planet’s orbit is called eccentricity, with 0 being a circle and 1 a straight line. Any value between 0 and 1 represents an ellipse. The shape of the Earth’s orbit is so close to being circular that the apparent size of the Sun does not appear to change as this animated graphic shows. The difference between perihelion and aphelion is about 3%.

   
   
   
   Eccentricity for each planet is listed below for comparison.

Planet	   Eccentricity	
Mercury	   0.2056
Venus	   0.0068
Earth	   0.0167
Mars	   0.0934
Jupiter	   0.0484
Saturn	   0.0542
Uranus	   0.0472
Neptune	   0.0086
Pluto	   0.2488

   To read more about the Earth’s orbit and get some teaching ideas click here to download a PDF copy of my January 2011 Scope on the Skies column “Solar Explorations“.
   

   
   
   

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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