Moon Slips Past the Scorpion

   Over the next two evenings, Saturday August 18th, and Sunday the 19th, the waning gibbous Moon will pass within about 8-10o from the reddish star Antares, the ‘heart’ of Scorpius the Scorpion.
   Joining the Moon and Jupiter are the planets Venus, Saturn, Mars, and Dwarf Planet Ceres.


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Perseids 2018 – A Good Year

   Before sunrise on Saturday August 11th, Sunday August 12th, and Monday August 13th look toward the east and southeast for ‘shooting stars’, or meteors. The short-lived streaks of light are radiating outward from the area of the constellation Perseus the Hero. These are the annual Perseids – one of the best meteor showers each year. And these three days are centered more or less on the peak.

   The Perseid Meteor Shower is named for the constellation from where the meteors radiate outward. This is the same for all meteor showers, and the ‘spot’ in the sky is known as the radiant. The Perseid radiant, as shown in the graphic, is within the Perseus constellation, and under ideal viewing conditions (dark and moonless skies) an average of about 60-80 meteors per hour could possibly be seen. This year without the interference of moonlight will increase the chances of seeing the meteors.

   The peak for this year’s Perseids is Monday September 13th at 1 UT (8 pm CDT) however Perseus and the radiant rise at around 11 pm local time. A couple of hours later should be high enough over the horizon to become visible. I always look for a triangle made from using the Pleiades open star cluster, the bright star Capella in the constellation Auriga, and a not as bright star, Mirfak in Perseus. The radiant is up to the left from Mirfak as the graphic shows.

   Best viewing times for seeing the Perseids are early morning a few hours before sunrise after Perseus has risen. This is an ideal time as the part of Earth you are viewing from is rotating toward the east, in the direction the Earth is revolving around the Sun. This means you will be seeing metaors ‘head-on’ as the enter the atmosphere.

    Meteor showers result from the Earth’s orbital path intersecting areas of comet debris. Comets, as they orbit the Sun, leave behind pieces of their icy, dirty, selves. If these debris clouds happen to be along the Earth’s orbital path then the Earth will regularly pass through the comet debris cloud. As this happens the small comet pieces hit our outer atmosphere and vaporize from the friction generated heat. We then see these as the shooting stars that make up meteor showers.


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July Full Moon at Apogee, Descending Node, A Lunar Eclipse, and Conjunction with Mars

Apogee Moon
   Our Moon reaches apogee, (greatest distance from Earth), on Friday July 27nd. At that time the Moon will more or less be at a distance of 31.84 Earth diameters (406,223 km or 252,415. miles) from the Earth.
   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*
*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?”

Descending Node
   Friday July 27th the full 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.

Total Lunar Eclipse
   When the Moon crosses the ecliptic, a node crossing, and the Moon is either at full or new phase there will be an eclipse. The length of the eclipse and whether or not it will be partial or total depends upon the timing. The closer the two events are to each other the greater the eclipse. This total lunar eclipse will be a long one at nearly 2 hours for totality. However the eclipse will not be visible from North America.
   Get eclipse information from the Hermit Eclipse web site.
   Watch the Lunar Eclipse live. Webcast hosted by the Bareket Observatory in Israel. Webcast starts at 18:30 UTC (1:30 pm CDT).

   On the day of the apogee and descending node the full Moon will be over the southern horizon and within about 6-7o from the planet Mars.


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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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It’s All About Exposure

   Last night after sunset I set out to try and capture a picture of Venus less then 1o from the open star cluster M-35, near the feet of the Gemini Twins. Additionally the first quarter Moon was within about 1.5o from the star Regulus, the ‘heart’ of Leo the Lion, and also easily seen as the bottom of the backward question shape this constellation is best known for. Since my western skies are illuminated by the lights from an athletic field, and the eastern suburbs of Kansas City, MO. the limiting magnitude is around 2 or 3, meaning that the dimmest stars easily seen in that direction have to be at least 3rd magnitude or brighter. So with that in mind I took over 60 pictures with various camera settings but the skies were just too bright to capture the light from the stars making up M-35.

   Then I turned my attention to the Moon and Regulus. Regulus was close enough to the Moon that it’s light was nearly washed out by the Moon’s reflected Sun light. The difficulty of catching both has to do with camera settings. For example opening the shutter and increasing the exposure time washes out the Moon but allows Regulus to be seen.
   I use a Cannon Rebel EOS T7i with a touch screen allowing me to change settings very easily and see the effect in the change in real time.
   Camera settings: 300 mm; f/13; 1/40 sec.; ISO-400

   For this picture I increased the exposure time but left the other settings as they were. The Moon is larger in this picture because I zoomed in on the original before cropping it for this blog.
   Camera settings: 200 mm; f/13; 5 sec.; ISO-400


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.

It’s the Autumn Equinox on Mars

   Tuesday May 22nd is the autumnal equinox on the planet Mars as the planet transitions from summer during its 684 Earth day orbit around the Sun.
   Seasons on Mars are marked by the planet’s heliocentric longitude coordinates using the position of Mars along its orbit around the Sun. At the Martian spring equinox Mars is at 0o longitude.
   Each seasonal start/ending point is 90 degrees apart, but because of an elliptical-shaped orbit each Martian season is of varying lengths. Mars is at its greatest distance from the Sun, aphelion, before it reaches the Martian summer solstice when Mars is at 70o longitude. Perihelion, its closest to the Sun, is when Mars is at 250o longitude.
   Eccentricity of Mars and Earth for comparison.
Mars: 0.0934 – Earth: 0.0167

   I’m not exactly sure why this particular date is used but by international agreement astronomers have selected 11 April, 1955 as 0 degrees for year 1 of this Martian calendar. What this means is that on Tuesday May 22nd, Earth time, it is the start of autumn of year 34 using the aforementioned calendar system.

Year 34
0 degrees — Spring Equinox — May 05 2017
90 degrees — Summer solstice — November 20 2017
180 degrees — Fall Equinox — May 22 2018
270 degrees — Winter Solstice — October 16 2018

Year 35
0 degrees — Spring Equinox — March 23 2019
90 degrees — Summer solstice — October 08 2019
180 degrees — Fall Equinox — April 08 2020
270 degrees — Winter Solstice — September 02 2020

Year 36
0 degrees — Spring Equinox — February 07 2021
90 degrees — Summer solstice — August 25 2021
180 degrees — Fall Equinox — February 24 2022
270 degrees — Winter Solstice — July 21 2022

   Learn a little (or a lot) more about the exploration of Mars at the NASA Journey to Mars web site.

   Learn a little (or a lot) more about Mars at the NASA/JPL Mars Curiosity mission web site.
   Or read about the InSight mission currently on its way to Mars.


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.

Sun Not Really in Gemini

  According to the pseudoscience of astrology the Sun enters the constellation of the Gemini Twins on Monday May 21st. When in fact the actual position of the Sun on this date is still within the boundary of the constellation of Taurus the Bull, as this graphic and the banner graphic show.

   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 more observing information for this month.