2016 Mars Autumnal Equinox

   Monday July 4th 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 Monday July 4th at 16 UT (11 am CDT) Earth time, is the start of autumn for Martian year 33 using the Earth-designed Martian calendar system.

   Mars is currently visible in the evening skies over the southern horizon after sunset local time.

Year 33
0 degrees — Spring Equinox — Jun 18 2015
90 degrees — Summer solstice — Jan 03 2016
180 degrees — Fall Equinox — Jul 04 2016
270 degrees — Winter Solstice — Nov 28 2016
Year 34
0 degrees — Spring Equinox — May 05 2017
90 degrees — Summer solstice — Nov 20 2017
180 degrees — Fall Equinox — May 22 2018
270 degrees — Winter Solstice — Oct 16 2018

Learn a little (or a lot) more about Mars at the NASA/JPL Mars Curiosity mission web site.

Here is approximately 3 minutes worth of Mars from the Orbit performance.

   
   
   

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.

It Is Okay To Point

clicl on picture to see it larger   During the lunar eclipse the other evening (on my side of the world) I had, for a time, the company of my daughter, granddaughter, wife, and our dog Tyler. As my daughter pointed toward the Moon and helping our granddaughter aim her binoculars I had this cosmic thought. Am I the only one wondering “if the Moon is there where is the Sun?” Obviously more or less 180o away from where the Moon is. After all this is a full Moon. So by putting the Moon behind you point toward the ground at an angle that is the same as the angle the Moon is above the horizon. That is where the Sun is at that moment.
camera-points    A look at how I had my camera setup sort of conveyed that idea as well.
   The Moon is within the constellation of Pisces near the vernal equinox, the crossing between the ecliptic and celestial equator. This means the Sun, if opposite from the Moon, would be at a similar crossing but the opposite season in the constellation of Virgo the Harvest Maiden – the autumnal equinox.

   Some pictures of the lunar eclipse around the center of my universe.

   
   
   
   

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.

December Solstice 2013

sagittarius-dec21   On Saturday, 21 December, at 11:11 CST (17:11 UT) the Sun will have reached its southernmost distance (in degrees of latitude and/or declination) from the Earth’s surface and celestial equator. Using Geographical coordinates the Sun is over the Tropic of Capricorn at a latitude of 23.5o South. Using Astronomical coordinates the Sun is -23.5o South (declination) and 18Hours RA (right ascension).

December Solstice
seasons   In its annual trek around the Sun the Earth reaches certain orbital positions that define the beginning and end for each of our four seasons. In either hemisphere, northern or southern, we learn these as the spring equinox, summer solstice, autumn equinox, and winter solstice. However the seasons are reversed or the opposite for each hemisphere. Winter in the northern hemisphere, for example, starts when summer in the southern hemisphere begins, and so on. This gives rise to the idea that it may be more practical to simply refer to these dates by the month that it occurs in. So for example rather than saying this is the “winter solstice” and then specifying which hemisphere you are referring to folks in either hemisphere would know which solstice, equinox, and or season it is by simply stating the name of the month for that event, i.e., December Solstice, March Equinox, June Solstice, September Equinox.

Along the Ecliptic
   How are the dates for these changes of seasons selected? As the Earth revolves around the Sun the Sun appears to move eastward against the stars in the background, regardless of the hemisphere. The apparent path the Sun follows is known as the ecliptic and in reality it is the orbit of the Earth superimposed on the stars in the background. seasons on ecliptic-aniAs a rule the starry sky is referred to as the celestial sphere, an imaginary sphere surrounding the Earth on which the ecliptic, stars, planets, and our Moon are located. The Earth’s equator and lines of latitude and longitude are likewise superimposed on the celestial sphere. This is a system similar to the use of latitude and longitude on the Earth’s surface that allows for the positioning or plotting of celestial objects including stars, galaxies, and closer to home the location of the Sun and orbiting planets.

   With regard to the Sun there are specific locations along the ecliptic that mark the change of seasons that correspond to geographic locations on the Earth’s surface. In the northern hemisphere we traditionally say that the Winter Solstice (change to December Solstice) (first day of winter) occurs when the Sun is south of the equator over the Tropic of Capricorn. The Spring Equinox (change to March Equinox) (first day of spring) is when the Sun is over the Earth’s equator. The first day of summer or the Summer Solstice (change to June Solstice) is when the Sun is north of the equator and is over the Tropic of Cancer. And finally when the Sun is back over the Earth’s equator it is the Autumn Equinox (change to September Equinox) and the start of autumn.

Mid-day Sun at 40oNorth

Mid-day Sun at 40oNorth

   These dates are more precisely determined by when the Sun has reached a specific astronomical coordinate on the celestial sphere. However without knowing the calendar date or astronomical position one can easily determine this date by observing the Sun’s apparent daily path regularly.
   In the northern hemisphere on the first day of winter during December the Sun will be south of the equator and will follow the shortest apparent path from east to west. It will rise in the southeast at its furthest south point from due east and set in the southwest at its furthest south point from due west. Midway between rising and setting the Sun will be at its lowest mid-day point above the southern horizon. In contrast it is the start of summer in the southern hemisphere during December. From those latitudes the Sun will follow its longest apparent path from east to west. The Sun will rise in the southeast at its furthest south point from due east and set in the southwest at its furthest south point from due west. Midway between rising and setting the Sun in the southern hemisphere would be at its highest mid-day point above the northern horizon.

The View From South of the Equator
   Keep in mind that these observations are based on the direction one typically faces to view the apparent path of the Sun follows between rising and setting. This direction is always toward or relative to the Earth’s equator and is centered mid-way between due east and due west. It is also latitude-dependent in that the length of daylight and night varies with latitude – more hours of daylight close to the equator where the Sun’s apparent path takes it higher above the horizon and subsequently giving more time above the horizon. Moving away from the equator the Sun’s apparent path doesn’t take it as high above the horizon and thus less daylight time. Nonetheless those in the northern hemisphere face toward south while those in the southern hemisphere face toward north. Remember – the Sun and all other celestial objects always rise in the east!

   
   
   
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.

September Equinox

Click on graphic to see it full size.

Click on graphic to see it full size.

   On Sunday 22 September at 3:44 pm CDT the Sun will have reached the astronomical coordinates of 0 degrees declination and 12 hours of right ascension, or RA. This places the Sun within the boundaries of the constellation Virgo the Maiden, or as some would say, “the Sun is in Virgo.”
Click on graphic to see it full size.

Click on graphic to see it full size.

   Declination is the astronomical equivalent to latitude measuring from 0 degrees at the equator to 90 degrees at either pole. Right ascension, or RA, is like longitude except that there is only east RA. The globe is divided into 24 sections, and like meridians of longitude, these hour circles are 15 degrees wide at the celestial equator and taper to a ‘point’ at the north and south pole respectively. In RA the ‘hour’ circles are counted from 0 hours to 23 hours. The 0 hour circle is at the intersection of the ecliptic and the celestial equator in the constellation of Pisces the Fishes.
Click on picture to see it full size.

Click on picture to see it full size.

   In a class lesson about seasons today would be one of the two days during the year when the Sun would be described as being over the Earth’s equator. If you were at the Earth’s equator the Sun would have an altitude of 90 degrees, or straight up in your sky at your local time for midday. At that moment there would not be a shadow. However at any other latitude, north or south at midday, the Sun would be at an angle less than 90 degrees and there would be a midday shadow. (Midday is the local time when the Sun is halfway between local rising time and local setting time. At any midday the Sun is at its maximum altitude above the southern horizon in the northern hemisphere, or is at its maximum altitude above the northern horizon in the southern hemisphere.)
   What is often most notable about an equinox day is the reminder that equinox means equal night as a reference to there being equal amounts of daylight, and night. Also on an equinox day the Sun would rise due east and set due west for virtually everywhere on the globe. The times for sunrise and sunset would be approximately 12 hours apart, and the rising time would be around 6 am local time, and the setting time would be around 6 pm local time.

Hola Moon doh

Hola ‘Moo’ndo! Think Globally.

   So why “September Equinox” instead of using the more familiar “Fall Equinox”. Primarily because the southern hemisphere is also changing seasons on this day however for the southern hemisphere this is the start of their spring season. Despite the opposite seasons it is somewhat of a northern hemisphere bias that traditionally we would call this day the “Autumnal or Fall Spring Equinox”, and in March we would say the “Spring” or “Vernal Equinox”. I favor the use of the name of the month so that regardless of which hemisphere it is just simply the March equinox or the September equinox, and by extension we would also have the June solstice and the December solstice..
   
   This short video shows students at Colegio Menor San Francisco de Quito, a school in Quito Ecuador, measuring the altitude of the sun hourly on the day of the 2004 September Equinox. They were taking part in Project SunShIP, Sun Shadow Investigation Project. There are also some pictures showing a local midday shadow from other participating schools in the United states.

   
   
   Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.