Wednesday morning December 5th the very thin 28-day old waning crescent Moon will be within 4-5o from the innermost planet Mercury. Venus shines very brightly above or west from the Moon. About an hour after the Moon and Mercury have risen the outer planet Jupiter will rise above the eastern horizon.
The pair should prove to be an interesting sight with binoculars or a wide-field telescope eyepiece. Venus shines with an apparent magnitude of -4.4, while Spica has an apparent magnitude of about 1.0.
Friday August 3rd all of the planets, except for Mercury, and some of the Dwarf Planets will be over the horizon during the hours before sunrise and the hours before sunset. The dwarf planets Pluto (14.2), Haumea (17.2), Makemake (16.7), and Eris(18.5) with low apparent magnitudes are too distant to be visible other than with larger aperture telescopes. However Dwarf Planet Ceres, at 8th magnitude could be visible with smaller telescopes and certainly with long exposure time imaging.
Feel the Heat?
Wednesday January 3rd at 5:35 UT (11:35 pm CST Tuesday January 2nd), 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 this year, 2018, the Earth is 0.98333 AU (147,099,586 km; 91,403,445 miles) (compared with last year 2017: 0.98331 AU (147,101,082 km; 91,404,374 miles) from the Sun. Approximately one-half year or one-half revolution later, on July 6th, the Earth is at aphelion and is 1.0167 AU (94,508,169 miles; 152,096,155 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.
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.
When the Earth’s orbital speed is compared to an outer planet there will be a time period when the faster moving Earth passes the outer planet. This sets up a temporary illusion where it appears as if the outer planet has reversed direction and is now moving backward toward the west, or in retrograde motion.
On the other hand the two inner planets, Mercury and Venus, also revolve around the sun toward the east and go through a period of retrograde motion. However theirs is not as a result of the Earth’s faster orbital speed but rather it is their faster orbital speed compared with the Earth’s that gives them their retrograde motion. And unlike the apparent retrograde motion for an outer planet, Mercury and Venus do actually move in retrograde motion as this animated graphic is showing.
So, on Saturday December 23rd the innermost planet, Mercury, ends its retrograde motion for this particular orbit and resumes its eastward motion. In the animated graphic the size for Mercury is exaggerated and the animation is set to a 1-day interval. It is showing Mercury moving westward as Venus is moving eastward. As Mercury moves along its orbit it eventually curves around and starts moving toward the east. This may be close to the day when Mercury reaches its furthest separation from the Sun on the west or right side of the Sun. That point is known as Greatest Western Elongation and is on January 1st. The counterpoint to this is when Mercury, or Venus, reaches their respective Greatest Eastern Elongation on the left or east side of the Sun.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.
This past week I had the privilege of working with two dynamic teachers and their Science classes in participating in the Sally Ride EarthKam Mission. As with previous missions the students work with a Google Earth type of map that shows the different orbital tracks the ISS will follow during the week. I remind them that is is sort of a ‘Forest Gump’ activity because like with the box of chocolates, you never know exactly what your picture will look like. How? Because of the weather or how accurately the location is selected. As a result of 176 requests 77 did not get taken and only 42 of the remainder of requests showed land features. The rest were clouded over.
Below are the pictures as a slide show. Hover the cursor over a picture to see any information from the requester. SL is Summit Lakes Middle School and WVE is Westview Elementary School. This may be followed by requester name and possibly name of picture request location.
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.
Sunday October 8th the innermost planet Mercury reaches superior conjunction – on the opposite side of the Sun from the Earth. For those that are curious, Mercury at superior conjunction is approximately 1.408 AU (130,881,776 miles; 210,633,801 km) from the Earth – the combined distance of the Earth to Sun distance plus the radius of Mercury’s orbit.
Mercury is not visible while in conjunction with the Sun but within the next week or so Mercury will reappear on the east side of the Sun and start becoming visible over the western horizon at sunset.