Sun Enters Scorpius

   Friday November 23rd the Sun, in its apparent eastward motion along the ecliptic, moves out of the constellation Libra the Scales and into the constellation of Scorpius the Scorpion. This is the true or actual position of the Sun as opposed to the pseudoscience of astrology which usually has the astrological Sun one constellation ahead or east from the Astronomical Sun’s position.

   Read a little more about how astrology has the Sun incorrectly placed in a previous blog, and in another blog discussing the effects of precession.
   
   
   

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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Sun Not in Sagittarius

  According to the pseudoscience of astrology the Sun enters the constellation of Sagittarius the Archer on Thursday November 22nd. When in fact the actual position of the Sun is still within the boundaries of the constellation of Libra the Scales.

   Read a little more about how astrology has the Sun incorrectly placed in a previous blog, and in another blog discussing the effects of precession.
   
   
   

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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Venus at Inferior Conjunction

   Friday October 26th the inner planet Venus reaches its orbital position known as inferior conjunction.
   This is one of four points along an inner planet orbit. At inferior conjunction Venus is between the Earth and the Sun – sort of like our new Moon phase – but not necessarily directly in line. If that were so, inferior conjunction at the same time as Venus in direct line with the Earth and Sun (a node crossing at inferior conjunction) it would be a somewhat rare transit of the Sun by Venus. These happen as a pair of transits about one time each century with the last pair of Venus transits in June 2004 and June 2012. Here are some pictures of the June 2012 Venus Transit. Here is a link to the Vimeo web site to watch a video of the 2012 Venus Transit.
   By the way the next pair of Venus Transits are December 2117, and December 2125.


    At this inferior conjunction Venus is 6.36o south of the ecliptic.

   
   
   
   

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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Saturn at East Quadrature – 2018

   Tuesday September 25th the position of the planet Saturn with respect to the Earth and the Sun places this ringed planet at what is called eastern quadrature. Saturn is at a 90 degree angle from us as this graphic shows. Think first quarter Moon as that is a fair comparison of the relative positions. At this position Saturn follows the Sun across the sky from east to west as the Earth is rotating, meaning that Saturn rises after the Sun and sets after the Sun.

   Where is Saturn now? Saturn is over the southern to southwestern horizon at sunset and is east (to the left) from the reddish star Antares in the constellation Scorpius the Scorpion. Further toward the west is the planet Jupiter, and to the east from Saturn is the planet Mars. And the 16-day old waning gibbous is above the eastern horizon.


   Saturn currently has a 0.47 apparent magnitude and is centered on the Milky Way within about 2o from a few easy to see Messier Objects. These include M-21 a 7th magnitude open star cluster; 5th magnitude M-8, the Lagoon Nebula; and 5th magnitude M-20, the Trifid Nebula.

   
   
   Learn a little (or a lot) more about Saturn by visiting the Cassini at Saturn mission web site. Click here to go to the Cassini Mission web site.

   This is a short 5 minute video I made as part of a live musical performance called “Orbit” that I was part of in May 2011. This is a piece from the much longer tour of the solar system performance and video and shows Saturn and some of its moons as viewed from the Cassini spacecraft that 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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Mars as Big as the Full Moon – But Only on Internet

   Tuesday July 31st the outer planet Mars will be at its closest to the Earth – this time around. Coincidentally this is 4 days after Mars was at opposition, and about 3 weeks after Earth was at aphelion. However this is not the closest Mars has been from the Earth. The closest, so far, was during August 2003.

A Martian Urban Legend
   “This year, during August, Mars will be at its closest to the Earth; so close, in fact, that it will appear as large as the full Moon.”
   Have you heard this urban legend before? Since 2003, emails describing this myth have circulated the internet usually during July or August. It all started during the opposition of August 2003 when Mars was about as close as it can ever be to the Earth (55,755,723 km – 34,645,000 miles) in a 20-year cycle of varying distances at opposition. Since then, usually around August, misinformation about the appearance of Mars circulates around the internet.

   In the time that it takes the Earth to catch up with and move into an opposition position, the outer planets have also been moving eastward relative to the stars in the background. Over time, the constellation where the outer planet is located during opposition and its retrograde loop gradually shift to the east. An opposition is also near the time when the distance between the Earth and an outer planet is around the minimum distance. Keep in mind that because the shape of the orbit is elliptical, this minimum distance will be different each time.

   However, there is an even more conclusive way to show that Mars could never be so close that it would appear as large as the Moon. To see how requires and some basic arithmetic. You will see that Mars could not appear to be as large as our Moon unless some force somehow caused Mars to change its orbital position and literally move closer to Earth.

   On July 31st, Mars will be 57,595,180 km (35,787,986 miles) from Earth. Mars is 6,792 km (4,220 mi.) in diameter. The Moon is 3,475 km (2,159 mi.) in diameter and is an average 384,400 km (238,900 mi.) from Earth. In terms of apparent size our Moon has an angular diameter of 0.5o (30 arc minutes or 1800 arc seconds), while on July 31st Mars has an apparent diameter of 24 arc seconds.
    So with the following calculation using those values (24 / 1800) x 100% = 1.3%) Mars is only 1.3% the apparent size of our Moon.

   Another method to calculate how large Mars will appear as compared to the Moon:
   (Mars’s diameter ÷ Mars’s distance) ÷ (Moon’s diameter ÷ Moon’s distance)

         (4220 / 35787986) / (2159 / 238900) = 0.013 = 1.3%
    In other words, Mars on 31 July will be only 1.3% of the apparent size of 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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Mars at Opposition – 2018

Mars at Opposition   Friday July 27th Mars will reach a point in its orbit around the Sun where it is at opposition relative to the Earth. at opposition The Earth is between the Sun and Mars, or any of the outer planets. At opposition both the Earth and the planet at opposition will have near identical heliocentric longitude. The opposition of Mars sometimes happens around the time that Mars is at its respective perihelion, closest to the Sun. If opposition happens during or near when the Earth is at its respective aphelion, furthest from the Sun, (first few days of July) then Mars will appear larger relative to when these dates are further apart.

What is opposition?
orbital-positions   The outer planets reach opposition when the Earth has moved into a position with the Sun on one side and the outer planet on the other side. Because all planets orbit in the same direction (toward the east), and all follow orbits that are slightly more elliptical than circular, oppositions occur at regular intervals of about 12 months (except for Mars). Mars is considerably closer to Earth and is moving faster than the other outer planets, so it takes approximately 26 months for Earth to catch up with Mars for an opposition.

   
   
   

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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Earth at Aphelion – 2018

Earth at Perihelion   Friday July 6th, 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 17 UT (12 pm CDT) on Friday July 6th the Earth is 1.01670 AU (94,508,169 miles; 152,096,155 km) from the Sun.
   Approximately one-half year or one-half revolution earlier, on January 3rd, the Earth was at perihelion, its minimum distance from the Sun for this year (0.98329 AU (91,402,516 miles; 147,098,090 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 return to bobs-spaces.


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