Which Way is North? Part 3

The Magnetic Compass
compass   A compass used for navigation is essentially a magnetized needle or bar mounted on a point so that it may freely pivot or swing around in any horizontal direction. In the Northern Hemisphere, the north-seeking end of the needle swings around to align itself parallel with a magnetic line of force, and in doing so, points toward the north magnetic pole. Use the link “Magnetic Effects in Space” in the Internet Resources below to watch astronaut/scientist Owen Garriott on a Skylab mission use a bar magnet to demonstrate the Earth’s magnetic field and lines of force.
   Because the north magnetic pole is not located where the true North Pole (geographic) is located, an adjustment called magnetic declination is made. The magnetic declination is the horizontal angle difference between geographic north and magnetic north. Using the magnetic-field calculator at the NOAA website (see Internet Resources below), you can easily determine the magnetic declination for your location, or any other location on Earth, for that matter. By comparing different locations and dates it becomes apparent that the angle for declination not only varies daily as the north magnetic pole migrates, but there is also a difference in magnetic declination based on both latitude and longitude. For example, at the latitude and longitude of Cambridge Bay, the magnetic declination is 8.07º; using the coordinates for Washington, DC, 39º N, 77º W, the magnetic declination is -10º51’. Negative magnetic declination values indicate that the north magnetic pole is to the west of the location, while positive values have the magnetic pole to the east of the location. From the nation’s capital, the north magnetic pole is toward the northwest, while from Cambridge Bay it would be to the northeast.

   Use this link to see a list of magnetic pole locations from 1590 through 2015 at the NOAA web site.
   Using the years above and this link to see an interactive world map where you may set the date to see the lines of magnetic declination for that time period.

compass   As previously described, a compass has a freely spinning needle balanced on the point from which it spins. When held flat in one’s hand, the needle quickly swings back and forth as it aligns itself with the magnetic lines of force. At lower latitudes, such as those within the continental United States, the compass needle mostly swings left or right as it aligns parallel with the magnetic field and lines of force. As one approaches the magnetic pole, the needle is still parallel with the magnetic lines of force and responding by swinging left or right. However, near the poles, the angle of the magnetic lines of force, with respect to the Earth’s surface, are now approaching vertical; as a result, the compass needle is pulled downward, rather than moving side to side, as would be the case at lower latitudes away from the magnetic pole. In effect, the downward pull on the needle creates increasing resistance on the formerly freely swinging horizontal motion the needle had at lower latitudes. At some point, the needle simply stops moving from side to side.

(Adapted from my January 2013 Scope on the Skies column)

Previous: Which Way is North – Part 1?
Previous: Which Way is North – Part 2?

Internet Resources
Magnetic effects in space—http://archive.org/details/skylab_magnetic_effects
Magnetic field calculators—www.ngdc.noaa.gov/geomag- web/#declination
The Magnetic Sun—http://solar.physics.montana.edu/ypop/Spotlight/Magnetic
Motion of the magnetic pole—http://image.gsfc.nasa.gov/poetry/activity/s8.htm
Planetary magnetic fields PowerPoint—http://education.gsfc.nasa.gov/nycri/units/pmarchase/3b-magnetic_field.ppt


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.

June Solstice

Sun's Apparent Motion Along the Ecliptic

Sun’s Apparent Motion Along the Ecliptic – from Taurus to Gemini

   Northern hemisphere spring comes to an end and its summer begins at 12:04 am CDT (05:04 UT) on 21 June as the Sun ‘reaches’ the celestial coordinates of 23.5oN and 6 hours right ascension. With respect to the Earth’s surface the Sun is described as over the Tropic of Cancer, 23.5oN of the Earth’s equator. At this same time the Sun is still within the boundaries of the constellation Taurus the Bull – but just barely. Interestingly 9 hours later, (9:00 am CDT – 14 UT), the Sun ‘will move’ into the region of Gemini as it crosses the boundary between Gemini and Taurus.
   We know that it is the Earth’s orbital motion around the Sun giving rise to the sun’s apparent eastward motion amongst the stars in the background. This is how the Sun ‘reaches’ a celestial coordinate, how it ‘crosses’ the boundaries between constellations, or how it is ‘in‘ a constellation.
   With respect to the southern hemisphere this is the end of their summer and start of their fall season so thinking globally my preference has been to use the name of the month to designate the season change. Hence the use of the term June Solstice rather than the limited to northern hemisphere term summer solstice.

   Follow the seasons by observing how vegetation changes during 1 year. The video below was produced by an Earth orbiting satellite operated by the NASA/NOAA Suomi National Polar Orbiting Partnership (NPP). It is a really interesting narrated tour of the Earth from orbit over a variety of geographic features and landscapes.

Just had to include this!!

Just had to include this!!


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

Blowing in the Wind

Satellite Image of Hurricane Sandy

   Hurricane Sandy, described as “historic” because of how late in the year it formed and its intensity, is approaching the eastern seacoast of the U.S. threatening our friends and family living in the storm area. They as well as everyone living in the area are certainly in our thoughts and prayers.
   In a recent post I shared some links for tracking hurricanes. So in that spirit here are some web links for tracking Hurricane Sandy. However the focus of these two are the winds that blow across our country as well as those that circle counterclockwise around the low pressure in the storm’s center.
   The Weather Underground web site, a longtime Internet presence, offers a static map (updated regularly) showing the wind patterns across the United States through the use of a color code for wind speed, and small arrows indicating the direction the winds are blowing toward. Other links on this page connect with the other display choices.
   The Wind Map web site offers a very unique view of the blowing winds across the United States with an animated map that is color coded to show wind speeds. From the animation it is obvious which direction the winds are blowing towards. This web site is one of the many visualization projects created by Fernanda Viégas and Martin Wattenberg two visualization artists at Google. Click here to learn more about them and their creative work.

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

Tracking Hurricanes

Watching the weather news coverage of Hurricane Isaac had me go off in search of software or online resources for tracking this hurricane as well as past hurricanes. There are computer programs, and apps for smartphones however I decided to stick with online resources. So with that in mind here a few of the many web sites that will allow for observing hurricanes whether it is for home/personal use or for use with students in the classroom.

Screen Shot

TV Station WRAL in Raleigh NC maintains an interactive hurricane tracking and modeling web site as shown in the graphic.
On NASA’s Hurricane resource web site there are many links under the Educator’s page leading to videos, podcasts,posters, and other resources relating to hurricane study as conducted by NASA.
Faculty at Pennsylvania State University have developed a hurricane tracking web site, the Real-time Atlantic Hurricane Forecast, where a variety of maps and graphics are displayed showing location and path of each hurricane selected.
The National Hurricane Center, of the National Weather Service, is probably the ‘goto’ web site for things relating to hurricanes, or weather in general.