July Moon at Descending Node

july8-descending-node   Wednesday July 8th at 00:09 UT, (Tuesday July 7th 7:09 pm CDT), our 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.

   The Moon is currently in its waning phases and is a few hours away from its last quarter phase. At sunrise local time watch for the Moon to be over the southern horizon, or the northern horizon if viewing from the southern hemisphere.

   
   
   
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.

So Close, Yet So Cold!

not drawn to scale

not drawn full size

   Today, Tuesday 1 January 2013, 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, at 11 p.m. CST (0500 UT 2 January) the Earth is 147,098,161 km from the Sun. Approximately one-half year or one-half revolution later, on 5 July, the Earth is at aphelion (152,097,427 km), its maximum distance from the Sun for 2013. 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.
   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.

   Caveat: “The above comments are obviously northern hemisphere biased, and I make no apologies to those in the opposite hemisphere who may be sitting on a beach somewhere enjoying strawberries and cream.” He said with a big grin.
   
   
Click here to go to the Qué tal in the Current Skies web site for more observing information.

On Top of the World

No, It's Not upside Down!

No, It’s Not upside Down!

   For the past week we have not seen the Sun much – even nights have been cloudy, and the temperature has been below freezing for more than a week. So combining those conditions with thoughts of warmer climes, and recent correspondence (aka e-mail) with a long-time friend living in New Zealand brought back memories of a trip I took with my wife, Sue, to New Zealand to attend an Astronomical Conference in New Zealand.
   Remember the 80’s song by Men at Work that asked the musical question “Do you come from a land down under?” That lyric gained new meaning for me while at that conference. I was politely corrected when I said that I came down to New Zealand. “No, no,” my hosts said, “You came up to New Zealand.”
   Have you ever wondered why we refer to folks from Australia and New Zealand as being from down under? How did their neck of the woods become down under and ours, conversely, up top? How is it that north is assumed to be at the top of the Earth and south at the bottom of the Earth? For that matter, is there really a top or bottom to the Earth?
   Try this: Hold a ball in one hand. Touch the top of the ball with your other hand. Directly opposite this point is the bottom of the ball. Turn the ball around in random directions a few times and again touch the top of the ball. Turn the ball around in random directions a second time and again touch the top of it. Did you touch the same spot on the ball each time? Was the bottom the same each time? So where would the top or the bottom of the ball be? Think of the ball as the Earth and each of the spots that were the ‘tops’ as different places on the surface of the Earth.
Nosotros casa es su casa

Nosotros casa es su casa

   Now try this: Take either a reticulated Earth globe or a globe that can be taken off of its mounting and set it in a large cup or bowl that will serve as the base. Adjust the globe so that your location on Earth is at the top. Mark this location on the globe with a straight pin or matchstick held with putty. Next, rotate the base so that globe’s north pole is aligned with true north and it’s south pole with true south. The Earth globe is now positioned with respect to the Sun exactly as the Earth is positioned in space with respect to the Sun. (Note that the shadow of your marker will mirror the shadows of all other objects cast on Earth with regards to compass direction.) From this ‘space’ perspective, you are clearly at the top of the Earth. Notice what point on the surface of the Earth is directly opposite your location, or at the bottom of the globe.
   If there is land at that location, imagine your counterpart doing this same activity. Who do you think your counterpart would consider to be at the bottom?    Imagine doing this for other locations around the Earth. What does this tell you about where the top and bottom of the Earth is?
   Your answer would have to be based on your frame of reference (you are on top wherever you are). This, unfortunately, is not the traditional way of looking at things around the globe.
"Be Careful Opening the Overhead Bins"

“Be Careful Opening the Overhead Bins”

   It has been a common and accepted convention to think of north as being at the top and south at the bottom. This thinking has established our perceived understanding of our place on the Earth and the vocabulary associated with this line of thinking.
This Northern Hemisphere–bias has even influenced how many astronomy books depict the Moon and constellation patterns. To someone from south of the equator, constellations and Moon phases are upside down according to the Northern Hemisphere–based star maps.
   So, is there actually a top or a bottom to the Earth? It’s up (or down) for you to decide.

   Thanks to Eric Jackson for planting these ideas in my head!
   
   
   
Click here to go to the Qué tal in the Current Skies web site for more observing information.