Saturn, You Are So Far Away!

click on graphic to see it larger
   Tuesday April 16th the outer ringed planet will be aphelion, its furthest distance from the Sun for this particular orbit. At aphelion Saturn will be approximately 10.0657 AU (1,505,807,287 km; 935,665,269 miles) from the Sun. Sort of splitting the difference Saturn is approximately 9.722 AU (1,454,390,499 km; 903,716,358 miles) from the Earth.
   Saturn is visible over the southern horizon before sunrise local time. It is near the ‘red planet’ Mars to the east, and Saturn is east from the heart of Scorpius the Scorpion, the reddish star Antares, and even further to the west is another outer ringed planet, Jupiter.

   
   
   

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.

Earth at Perihelion 2017

Earth at Perihelion   Feel the Heat?
   Wednesday January 4th at 14 UT (8 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 the Earth, this year, is 0.98330 AU (147,101,082 km; 91,404,374 miles) (compared with last year 2016: 0.98331 AU (147,099,586 km; 91,403,445 miles) from the Sun. Approximately one-half year or one-half revolution later, on July 3rd, 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 2017. 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.

Earth at 2016 Aphelion

Earth at Perihelion   Monday 4 July, 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 16 UT on Friday 4 July (11 am CDT) the Earth is 1.01675 AU (94,512,817 miles; 152,103,635 km) from the Sun.
   Approximately one-half year or one-half revolution later, on 4 January, the Earth is at perihelion, its minimum distance from the Sun for the year (0.98330 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 go to bobs-spaces.

Chill Out! We’re Halfway There, Or Are We Halfway After?

   Saturday July 2nd at 0 UT (19 CDT Friday July 1st) the Earth will be at the middle of the year, halfway around the Sun and halfway through the year for this particular orbit. This begins the second half of the year, and July 2nd, the 182nd day of the year, according to some calendars, is also known as Hammock Day. So chill out!
    Coincidentally Earth will be at aphelion, its most distant point from the Sun, on Monday July 4th. This is shortly after the first day of summer in the northern hemisphere, and of course the first day of winter in the southern hemisphere. //Darn that axial tilt//

   
   
   

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.

Mercury at Aphelion

mercury at aphelion   Sunday February 21st the innermost planet Mercury reaches aphelion, its furthest distance from the Sun, 0.4667 AU, (69,817,326 km; 43,382,475 miles). Interestingly, I suppose, is that the next time Mercury reaches aphelion will be May 19th, 10 days after the Mercury transit of the Sun.

   All planets orbit the Sun with an orbital shape that is not circular but rather elliptical. How much away from being a circle is called eccentricity of an orbit. With 0 representing a circle and 1 a straight line Mercury, with an eccentricity of 0.2056, has the greatest eccentricity of the other 7 planets. The dwarf planet Pluto, for comparison, has an eccentricity of 0.2488.

   
   
   
   

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.

A Martian Urban Legend

   It’s back!
   A friend asked about this picture and from the text on the graphic I am assuming that this is the Internet Urban Legend that comes back around this time of the year. This is the suggestion that Mars will be so close to the Earth that it will appear as large as the full Moon. Nothing of the sort will happen. This idea probably got its start from August 2003 when Mars was at opposition very close to the time it was at perihelion, closest to the Sun, while the Earth was at aphelion, its most distant from the Sun. This combination brought the Earth and Mars to about as close as they ever could be.

   For the record the Moon on August 27th is in the waxing gibbous phase, 2 days before full Moon, and it rises about 2 hours before sunset. Mars, on the other hand, rises about 2 hours before sunrise with the much brighter Venus nearby.

    I posted a blog about this previously, as well as wrote one of my Scope on the Sky columns about this suggestion.

   
   
   
   

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 Perihelion 2015

jul6-earth-aphelion   Monday 6 July, 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 20 UT on Monday 6 July (3 pm CDT) the Earth is 1.01668 AU (94,506,310 miles; 152,093,163 km) from the Sun. Approximately one-half year or one-half revolution later, on January 2nd, the Earth is at perihelion and is 0.98330 AU (91,403,445 miles; 147,099,586 km), its minimum distance from the Sun for the year. This 3% 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.