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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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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Jupiter – Mercury Conjunction

   Friday and Saturday mornings, December 21st and 22nd, the outer planet Jupiter and the inner planet Mercury will be within a couple of degrees from each other. Both rise about an hour before sunrise local time and 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.


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Mercury at Western Elongation

   On Saturday December 15th Mercury, the innermost planet, will reach its orbital position known as greatest western elongation at 21.3o. 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.

   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. Venus, to the west from Mercury rises about 2 hours before Mercury, and Jupiter rises shortly after Mercury.

   
   
   
   
   

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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Mars – Neptune – Moon in Conjunction

   Friday evening December 14th the first quarter Moon, Mars, and Neptune will all be 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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