Earth at 2019 Perihelion

Earth at Perihelion   Feel the Heat?
   Thursday January 3rd at 3 UT (10 pm CST Wednesday January 2nd), as the Earth continues its annual trek around the Sun, the Earth reaches a point in its orbit that is called perihelion. Perihelion is the minimum distance that separates the Earth from the Sun, and we are the closest to the Sun for the year at this point in the orbit. So this year, 2018, the Earth is 0.98333 AU (147,099,586 km; 91,403,445 miles).
   Approximately one-half year or one-half revolution later, on July 4th, the Earth is at aphelion and is 1.0167 AU (94,508,169 miles; 152,096,155 km), its maximum distance from the Sun for 2019. This difference 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.
   Here is a good classroom activity about the Earth’s orbit and its effect on the apparent size of the Sun: Why Does the Size of the Sun Appear to Change? A Year of the Sun.

   
   
   

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.

Saturn at 2018 Solar Conjunction

10dec-saturn-solar-conjunction   Wednesday January 2nd the planet Saturn will have reached the astronomical coordinates that officially place it at solar conjunction. From our perspective the planet is behind the Sun, or on the opposite side of the Sun from the Earth.
   In reality it is not as much as Saturn moving behind the Sun as it is the Sun passing in front of Saturn – or so it seems. As a distant outer planet Saturn moves more slowly around the Sun than the Earth does. One year on Saturn is equal to 29.7 years (10,832 days) on Earth. So in one day Saturn would travel how much of the 360o orbit around the Sun? That would amount to approximately 0.033o each day.
   The Sun, in its apparent motion along the ecliptic moves at the rate the Earth is moving which is 0.99o each day. So with the Sun’s apparent motion (0.99o/day) it quickly, relative to Saturn, passes Saturn while both are moving eastward. So with that in mind you could start watching for Saturn to reappear in the morning skies later next month.

   
   
   

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 Conjunction with Venus and Ceres

   Tuesday morning January 1st the thin 25-day old waning crescent Moon (apparent magnitude -10.96) will be within 3-4o from the inner planet Venus (apparent magnitude -4.47) and 1-2o from the Dwarf Planet Ceres (apparent magnitude 8.0). All three will fit within the field of view of 7×50 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.


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A Trio of Pairs

   Sunday morning December 30th the 23-day old waning crescent Moon (apparent magnitude -11.76) will be 5-6o from the blue-white star Spica in the constellation of Virgo the Harvest Maiden. Both the Moon and Spica (apparent magnitude 0.96) will fit within the field of view of 7×50 binoculars.

   Rising about an hour after the Moon and Spica is another pair of celestial objects, the hard to miss Venus (apparent magnitude -4.48) is within 2-3o from the Dwarf Planet Ceres (apparent magnitude 8.0). Both of these will easily fit within the field of view of 7×50 binoculars.

   
   
   
   
   
   Wait about an hour, depending on your eastern horizon, and the planet Jupiter (apparent magnitude -1.8) will be about 5-6o from the reddish star Antares (apparent magnitude 1.0) in the constellation of Scorpius the Scorpion. And they will be high enough above the horizon to be more visible. Both Jupiter and Antares will 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.


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Christmas Morning with the ISS and the Moon

   Happy Holidays.
    This was the sky this morning with the International Space Station passing from west to the northeast and along the way passing my chimney and the North Star. The ISS, as it was rising, passed near the waning gibbous Moon, as the Moon was setting. However, the camera was aimed toward the North Star away from the Moon as the ISS past by 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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Moon Has A Heart – The Lion’s

   Tuesday evening and Wednesday evening December 25th and the 26th the 18 and 19-day old waning gibbous Moon will pass by the star Regulus. Regulus is the heart of Leo the Lion and shines with an apparent magnitude of 1.34.

   
   
   

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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December Moon at Perigee and Ascending Node

   Our Moon reaches perigee, (closest distance to Earth), for this orbit on Monday December 24th. At that time the Moon will be at a distance of 28.30 Earth diameters 224,352 miles (361,060 km) from the Earth.

   Monday December 24th the waning 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 December 24th the 18-day old waning gibbous Moon rises around 7:30 pm 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 go to bobs-spaces.


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