Last evening was my first opportunity to see the ISS in at least a month so I quickly set up my camera aiming it toward Venus and Mars. However my aim was off by quite a bit so I hurriedly readjusted the camera, 3 times, to capture these pictures. All were taken with ISO 800; F5; 18mm; 2.5 second. Pictures were stacked, merged, using Starstax.
Thursday January 12th the gas giant planet Jupiter reaches the point along its orbit around the Sun where Jupiter would be described as being at western quadrature. It is at a 90o angle relative to the Earth and the Sun.
Also on this day the inner planet Venus reaches a point in its orbit known as eastern elongation. This puts Venus at somewhere around a 90o angle relative to the Earth and the Sun when viewed looking down from above the solar system but Venus is on the other side of the Sun from where Uranus is currently located.
In terms of elongation Venus is 47.2o to the east of the Sun. In terms of viewing angle you are the point of a triangle with Venus at the left base corner and the Sun at the right base corner. The angle between the Sun and Venus is the elongation angle, and at 47.2o Venus sets around 3 hours after sunset as its seting motion or speed is (15o per hour based on Earth rotation rate.
Where is Jupiter currently? Jupiter rises around midnight local time and is located high above the southern horizon at sunrise local time near the blue-white star Spica in Virgo the Harvest Maiden.
And Venus? Can’t miss this planet as it is the brighest stellar object over the western horizon at sunset.
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.
On Sunday December 11th Mercury, the innermost planet, will reach its orbital position known as greatest eastern elongation. At that moment Mercury, the Sun, and the Earth, would be arranged in something close to approximating a right angle as this graphic shows.
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!
This animated graphic shows Mercury at its eastern elongation and its orbital path. The horizon is removed in one graphic so Mercury’s orbit around the Sun could be visualized better.
Wednesday November 9th the waxing gibbous Moon will be crossing the plane of the ecliptic moving south. This is known as the descending node, one of two intersections the Moon’s orbital path (dark green line) has with the ecliptic.
On the day of the node crossing the 10-day old waxing gibbous Moon rises around mid-day local time and is visible most of the night, setting a couple of hours after midnight.
The Moon reaches perigee, (minimum distance from Earth), this month on Sunday October 16th. At that time the Moon will more or less be at a distance of 28.05 Earth diameters (357,861 km or 222,364 miles) from the Earth.
The Moon reaches perigee Sunday at 23:47 UT (6:47 pm CDT) and this is less than 20 hours after it was at full Moon phase also on Sunday but at 4:23 UT, (11:23 pm CDT October 15th). Since the full Moon is this close to its closest to the Earth for this orbit the full Moon could be considered one of the ‘Super Moons’ this year.
Does our Moon actually go around the Earth as this graphic shows? From our perspective on the Earth the Moon appears to circle around the Earth. However, in reality, the Moon orbits the Sun together with the Earth*
On the day of the lunar perigee the 16-day old just past full waning gibbous Moon rises at around sunset local time and is over the southwest horizon at sunrise the following morning. Two of the Dwarf planets, Eris and Ceres are near the Moon but due to their respective apparent magnitudes (Ceres 7.0; Eris 18.6) and the bright reflected light from the Moon the two are all but invisible. Interestingly these two represent two similar but very different types of Dwarf Planets – Ceres is within the main asteroid belt at a distance of 1.9 AU (176,616,033 miles; 284,235,954 km) while Eris is in the outer regions of the solar system at a distance of 95 AU (8,830,801,690 miles; 14,211,797,715 km) from the Earth.
*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?”
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.
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“.
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//