Earth at Aphelion – 2018

Earth at Perihelion   Friday July 6th, 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 17 UT (12 pm CDT) on Friday July 6th 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 (0.98329 AU (91,402,516 miles; 147,098,090 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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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.

April Moon at Ascending Node

   Monday April 23rd the waxing gibbous Moon will be crossing the plane of the ecliptic moving north relative to the ecliptic. This is known as the ascending node, one of two intersections the Moon’s orbital path has with the ecliptic. The ecliptic is actually the Earth’s orbit and the Moon’s orbit is inclined about 6o from the ecliptic. So there are two node intersections, the ascending and descending nodes.


   On Monday evening April 23rd the 8-day old waxing gibbous Moon will be within the boundaries of Leo the Lion and about 7o west (right) from the ‘Heart’ of the Lion, the star Regulus.

   Does our Moon actually go around the Earth as many graphics show? 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 return to bobs-spaces.

A Week of Moon Conjunctions


   Over the next 7 days (mornings) the Moon, as it wanes toward last quarter, will pass closely by several planets and brights stars in some close and some not so close conjunctions.
   Perhaps the best morning will be on April 7th when the near last quarter Moon will be 1-2o from Saturn and about 4o from Mars.
All three will easily fit within the field of view of binoculars.

   
   
   
   
   
   
   
   
   
   

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.

March Moon at Ascending Node

   Tuesday March 27th the waxing gibbous Moon will be crossing the plane of the ecliptic moving north relative to the ecliptic. This is known as the ascending node, one of two intersections the Moon’s orbital path has with the ecliptic. The ecliptic is actually the Earth’s orbit and the Moon’s orbit is inclined about 6o from the ecliptic. So there are two node intersections, the ascending and descending nodes.


   On Tuesday evening March 27th the 10.5-day old waxing gibbous Moon will be within the boundaries of Leo the Lion and about 7o west (right) from the ‘Heart’ of the Lion, the star Regulus.

   Does our Moon actually go around the Earth as many graphics show? 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 return to bobs-spaces.

Jupiter Backs Up!

   Friday March 9th the outer ringed planet Jupiter stops its regular direct motion to the east and begins to move in retrograde toward the west. Retrograde motion for any planet further from the Sun then the Earth, that we observe from Earth, is an apparent motion of that planet caused by the faster moving Earth along its orbital path. As the Earth catches up and passes an outer planet that planet appears to move toward the west. Depending on the planet the retrograde motion may last a few weeks or months. Jupiter will be within the boundaries of the constellation Libra the Scales.

   Jupiter along with Mars and Saturn all rise before sunrise local time and the morning skies on March 9th the 21-day old last quarter Moon will be about 7o from the ‘red planet’ Mars.

   Follow the planet by way of its coordinates using just right ascension with the free equatorial star chart from Stephen F. Austin University.

Date Right Ascension
(Hours – Minutes)
June 5            14 52
June 15         14 48
June 25         14 46
July 5            14 45
July 15            14 44
July 25            14 46
August 4          14 48
August 14         14 51
August 24         14 55
September 3      15 00

   
   
   

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.

Mars Opposite Uranus

   Tuesday January 30th Mars and Uranus will reached their respective orbital positions that have them 180o apart. Mars will be located at approximately 207.6o and Uranus at approximately 27.6o of heliocentric longitude. This called heliocentric opposition.
   Interestingly at their last heliocentric opposition, February 26th 2016, Mars was located at approximately 200.0o and Uranus at approximately 20.0o of heliocentric longitude.
   
   
   

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.