Mars at Perihelion

mars-perihelion   Friday December 12th the planet Mars reached a point in its annual orbit around the Sun when it is the closest to the Sun. This is known as perihelion and for Mars it will be 1.3812 AU (206,624,579 km; 128,390,561 miles) from the Sun. An ‘AU’ is an Astronomical Unit, or the average Earth to Sun distance of 93,000,000 miles.
   Mars is tilted on its axis of rotation by about the same as the Earth so it too has a cycle of seasons. And coincidentally like the Earth, Mars is also closest to the Sun during its northern hemisphere winter. The Mars winter solstice is January 11th 2015.

   Take a brief tour of Mars and its two moons in a short video clip from a longer solar system tour video I made as part of a live musical performance at the Gottlieb Planetarium in Kansas City a few years ago.

   
   
   
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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.

So Close, Yet So Cold!

not drawn to scale

not drawn full size

   Today, Tuesday 1 January 2013, 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 11 p.m. CST (0500 UT 2 January) the Earth is 147,098,161 km from the Sun. Approximately one-half year or one-half revolution later, on 5 July, the Earth is at aphelion (152,097,427 km), its maximum distance from the Sun for 2013. 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.
   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.

   Caveat: “The above comments are obviously northern hemisphere biased, and I make no apologies to those in the opposite hemisphere who may be sitting on a beach somewhere enjoying strawberries and cream.” He said with a big grin.
   
   
Click here to go to the Qué tal in the Current Skies web site for more observing information.