Earth at its 2015 Perihelion

Earth at Perihelion   Feel the Heat?
   Sunday January 4th at 7 UT (1 am CST), 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, at 12:00 UT (0600 CST) the Earth is 0.98328 AU (91,401,586 miles; 147,096,594 km) from the Sun. Approximately one-half year or one-half revolution later, on 6 July, the Earth is at aphelion and is 1.01668 AU(94,506,310 miles; 152,093,163 km), its maximum distance from the Sun for 2015. 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.
   
   
   
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Caution: Objects viewed with an optical aid are further than they appear.
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

On the 4th Night

              Happy   4th of   July!!                     

jul4-aphelion   Friday 4 July is not only the day we in the United States of America celebrate our country’s Independence Day, it is also the day that the Earth reaches aphelion, its greatest distance from the Sun. For the record we are closest to the Sun, perihelion, around the beginning of January.
   So despite the summer heat, humidity, and a late sunset there will be more in the skies this 4th than just fireworks.
 At around sunset look low toward the western horizon for 3 stars arranged in a short diagonal line. No it is not Orion’s Belt. The one on the lower left side is the planet Jupiter near the Gemini Twin stars.

Click on graphic to see it full size.

Click on graphic to see it full size.

 The nearly first quarter Moon is over the southwestern horizon and is just to the west (right) from the planet Mars and the bluish-white star Spica.
 Both Mars and Spica are close enough so that they fit within the field of view of 7×50 binoculars.
 Look left from this pair and higher above the southern horizon for the planet Saturn, one of 4 planets in our solar system with rings.

Click on graphic to see it full size.

Click on graphic to see it full size.

   If your viewing area is under dark enough skies, away from the light-polluted metropolitan area, then look toward the eastern horizon for the glow of the Milky Way as it rises. The teapot shape asterism for Sagittarius is above the southern horizon throughout this month and if the Milky Way is visible it looks like steam rising from the teapot’s spot. This graphic shows the location of Dwarf Planet Pluto. Pluto is also at opposition. Opposition for an outer planet is an arrangement of objects like a full Moon in that at opposition an outer planet is on the opposite side of the Earth from the Sun.
   While it is too dim to see with the unaided eye or even binoculars it is a neat idea to think about what Pluto looks like. I say this (actually write) because around this time next year NASA’s New Horizon spacecraft will be flying past Pluto sending back our first truly good look at this very distant solar system object.

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Caution: Objects viewed with an optical aid are further than they appear.
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.

Earth at 2014 Perihelion

Earth at Perihelion   Today, Saturday 4 January 2014, 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, at 12:00 UT (0600 CST) the Earth is 0.98330 AU (91,403,445 miles; 147,099,586 km) from the Sun. Approximately one-half year or one-half revolution later, on 4 July, the Earth is at aphelion and is 1.01682 AU(94,519,324 miles; 152,114,107 km), its maximum distance from the Sun for 2014. 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.

   
   
Click here to go to the Qué tal in the Current Skies web site for more observing information.

Super Mini-Sun

Sunrise - 5 July

Sunrise – 5 July

   This Friday, 5 July, the Sun will be its smallest appearing and most distant for the year as the Earth reaches aphelion, the point in its orbit around the Sun where the Earth is at its maximum distance from the Sun. At 10 am CDT the Earth and Sun will be separated by 94,508,19 miles, (152,096,155 km). Conversely the Earth and Sun will be at their closest next year, perihelion, on 4 January 2014 when the two will be separated by 91, 405, 333 miles (147,103,000 km).
   The difference between the two distances is approximately 3 percent and is as a result of the Earth having a slightly elliptical shaped orbit. With a nearly circular orbit this in turn helps to explain that it is not distance from the Sun that is a reason for seasons. Seasons on Earth result from the Earth’s 23.5 degree tilt on its axis and its revolution around the Sun.
   Important note: the banner graphic at the top of the page is not actual size!

   Click here to go to the Qué tal in the Current Skies web site for more observing information.