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April 5, 2008Earth Unplugged!
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Archive for the ‘Come Blog With Us’ Category
The Butterfly Effect
March 23, 2008
Vernal Equinox
March 9, 2008Equinoxes and solstices
The ecliptic
During the course of a year the Earth completes one orbit around the Sun. To us on Earth we see this as the Sun moving against the background of stars through the year, along an imaginary line which we call the ecliptic. This defines the plane in which the Earth and most of the other planets orbit around the Sun. The directions to the north and south ecliptic poles are at right angles to this. The Zodiac is the band of constellations running along the ecliptic.
The celestial poles and celestial equator
The Earth’s orbit around the Sun takes 365.25 days, and a ‘day’ is, of course, defined as the Earth spinning once on its axis. The Earth’s axis of rotation, tilted at 23.5 degrees to the line of the poles of the ecliptic, gives us the directions to the north and south celestial poles. The bright star Polaris is currently showing us the direction of the north celestial pole. Like a spinning top this axis is precessing around the ecliptic pole with a period of 26,000 years.
The celestial equator is the projection of the Earth’s equator onto the sky. As the Sun moves in its apparent track along the ecliptic it is for half the year seen to be above the equator (northern summer) and half the year below the equator (northern winter). The Sun will therefore appear to cross the equator twice in a year.
The equinoxes
At the times when the Sun is crossing the celestial equator day and night are of nearly equal length at all latitudes and so we call these dates the equinoxes. In March, as the Sun is moving northwards along the ecliptic, this is called the vernal equinox and in September as the Sun is moving southwards we refer to it as the autumnal equinox. The equinoxes are also the points on the celestial sphere where the ecliptic and equator cross and the vernal equinox is used as the zero point in measuring star co-ordinates.
Why do the equinoxes not always occur on the same days each year?
The Earth takes approximately 365.25 days to go around the Sun. This is the reason we have a leap year every 4 years, to add another day to our calendar so that there is not a gradual drift of date through the seasons. For the same reason the precise time of the equinoxes are not the same each year, and generally will occur about 6 hours later each year, with a jump of a day (backwards) on leap years. The table below shows the dates and times of of both the vernal (spring) and autumnal equinoxes for a period of ten years:
|
Year |
Vernal equinox |
Autumnal equinox |
Leap year |
|---|---|---|---|
|
2000 |
20 March, 07.35 |
22 September, 17.27 |
yes |
|
2001 |
20 March, 13.31 |
22 September, 23.04 |
|
|
2002 |
20 March, 19.16 |
23 September, 04.55 |
|
|
2003 |
21 March, 01.00 |
23 September, 10.47 |
|
|
2004 |
20 March, 06.49 |
22 September, 16.30 |
yes |
|
2005 |
20 March, 12.33 |
22 September, 22.23 |
|
|
2006 |
20 March, 18.25 |
23 September, 04.03 |
|
|
2007 |
21 March, 00.07 |
23 September, 09.51 |
|
|
2008 |
20 March, 05.48 |
22 September, 15.44 |
yes |
|
2009 |
20 March, 11.43 |
22 September, 21.18 |
|
|
2010 |
20 March, 17.32 |
23 September, 03.09 |
|
|
all times are UTC (GMT) |
|||
In fact it is only after a complete leap-year cycle of four centuries that these dates will be repeated. In the present century the times of the equinoxes have ranged between the latest dates - March 21 at 19h and September 24 at 06h (in 1903) - to the earliest dates - March 20 at 08h and September 22 at 17h (in 2000).
Why do the dates of the vernal and autumnal equinox not occur on the days when day and night are equal?
A quick look at sunrise and sunset times on successive days in March and September will show that the above dates do not coincide precisely with the times of equal day and night length. There is another factor to be considered which is that the orbit of the Earth around the Sun is an ellipse and not a circle. By Kepler’s Law the Earth moves fastest when it is closest to the Sun (approximately 3 January each year) and slowest when it is furthest away (approximately 4 July ). This ‘unequal motion’ causes variations in the length of the solar day and in the times of sunrise and sunset.
The equation of time
The combined effects mean that the Sun does not cross the meridian (when it is highest in the sky) at precisely local noon each day. The difference between clock-defined noon and the time when the Sun is on the meridian is called the Equation of Time and represents the correction which must be applied to the time given by a sundial to make it agree with clock time. This correction can be as much as 16 minutes in either direction. For more information, see our fact file on the the equation of time.
The solstices
The times when the Sun is at its furthest from the celestial equator are called the summer and winter solstices and these occur in mid-summer and mid-winter.
|
Year |
Summer |
Winter |
Leap year |
|---|---|---|---|
|
2000 |
21 June, 01.48 |
21 December, 13.37 |
yes |
|
2001 |
21 June, 07.38 |
21 December, 19.21 |
|
|
2002 |
21 June, 13.24 |
22 December, 01.14 |
|
|
2003 |
21 June, 19.10 |
22 December, 07.04 |
|
|
2004 |
21 June, 00.57 |
21 December, 12.41 |
yes |
|
2005 |
21 June, 06.46 |
21 December, 18.35 |
|
|
2006 |
21 June, 12.26 |
22 December, 00.22 |
|
|
2007 |
21 June, 18.06 |
22 December, 06.08 |
|
|
2008 |
20 June, 23.59 |
21 December, 12.04 |
yes |
|
2009 |
21 June, 05.45 |
21 December, 17.47 |
|
|
2010 |
21 June, 11.28 |
21 December, 23.38 |
|
|
all times are UTC (GMT) |
|||
(Note: Don’t be fooled by the fact that during this period the summer solstice always occurred on 21 June. This is just a feature of the slow drift through the dates on a 400 year cycle mentioned above. The last time the solstice was not on this date was 1975 when it occurred on 22 June at 00.27 and in the year 2012 it will occur on 20 June at 23.09. For the dates in between the summer solstice will fall on 21 June.)
The apparently odd behaviour of sunrise/set times near the winter solstice
The winter solstice is the time when the Sun reaches its southmost distance from the celestial equator and hence, in northern latitudes is the day when the Sun is lowest in the sky at noon. This is, naturally, the shortest day of the year in northern latitudes. To many people it seems odd, therefore, that the time of sunrise continues to get later in the day after the solstice.
The period when the Equation of Time is changing fastest in the whole year is very close to the winter solstice. It changes by 10 minutes from 16 December to 5 January. This means that the time at which the Sun crosses the meridian changes by 10 minutes in this interval and also that the times of sunrise and sunset will change by the same amount.
Near the winter and summer solstices the Sun’s height in the sky changes very slowly and the length of the day also changes slowly. The rapid change due to the Equation of Time dominates the very slow change in day length and leads to the observed sunrise times.
What is the difference between Midsummer Day and the summer solstice?
Midsummer Day is June 24 (each year) and is one of the four Quarter Days in the Legal Calendar. The other Quarter Days are Lady Day (March 25), Michaelmas (29 September) and Christmas Day (25 December). The Summer Solstice is explained above.
Next Stop Israel
March 5, 2008
Class X01 is going to be studying Israel in Social Studies. Below are some links.
http://www.israelemb.org/kids/jerusalem.html
http://www.surfnetkids.com/jerusalem.htm
New Year Resolutions…
January 4, 2008
Penny Harvest Countdown…
November 28, 2007
Everything you ever wanted to know about turkeys… and then some..
November 23, 2007
Gobble, Gobble
November 18, 2007
Veteran’s Day
November 3, 2007
Now more than ever we should pay homage to this day. Rememberance of all those who give their lives so we can live in our great country.
Armistice Day Becomes Veterans Day
World War I officially ended on June 28, 1919, with the signing of the Treaty of Versailles. The actual fighting between the Allies and Germany, however, had ended seven months earlier with the armistice, which went into effect on the eleventh hour of the eleventh day of the eleventh month in 1918. Armistice Day, as November 11 became known, officially became a holiday in the United States in 1926, and a national holiday 12 years later. On June 1, 1954, the name was changed to Veterans Day to honor all U.S. veterans.
In 1968, new legislation changed the national commemoration of Veterans Day to the fourth Monday in October. It soon became apparent, however, that November 11 was a date of historic significance to many Americans. Therefore, in 1978 Congress returned the observance to its traditional date.
Tomb of the Unknowns
Official, national ceremonies for Veterans Day center around the Tomb of the Unknowns.
To honor these men, symbolic of all Americans who gave their lives in all wars, an Army honor guard, the 3d U.S. Infantry (The Old Guard), keeps day and night vigil.
At 11 a.m. on November 11, a combined color guard representing all military services executes “Present Arms” at the tomb. The nation’s tribute to its war dead is symbolized by the laying of a presidential wreath and the playing of “Taps.”
Unknown Soldier Identified
On Memorial Day (which honors U.S. service people who died in action) in 1958, two more unidentified American war dead, one from World War II and the other from the Korean War, were buried next the unknown soldier of World War I.
A law was passed in 1973 providing interment of an unknown American from the Vietnam War, but because of the improved technology to identify the dead, it was not until 1984 that an unidentified soldier was buried in the tomb.
In 1998, however, the Vietnam soldier was identified through DNA tests as Michael Blassie, a 24-year-old Air Force pilot who was shot down in May of 1972 near the Cambodian border. His body was disinterred and reburied by his family in St. Louis, Missouri.
Day of the Dead
November 3, 2007
History of Halloween
October 27, 2007
Halloween Safety Tips~~~
October 27, 2007
Red Cross Halloween Safety Tips for Kids and Adults
With witches, goblins, and super-heroes descending on neighborhoods across America, the American Red Cross offers parents some safety tips to help prepare their children for a safe and enjoyable trick-or-treat holiday. Halloween should be filled with surprise and enjoyment, and following some common sense practices can keep events safer and more fun.
- Walk, slither, and sneak on sidewalks, not in the street.
- Look both ways before crossing the street to check for cars, trucks, and low-flying brooms.
- Cross the street only at corners.
- Don’t hide or cross the street between parked cars.
- Wear light-colored or reflective-type clothing so you are more visible. (And remember to put reflective tape on bikes, skateboards, and brooms, too!)
- Plan your route and share it with your family. If possible, have an adult go with you.
- Carry a flashlight to light your way.
- Keep away from open fires and candles. (Costumes can be extremely flamable.)
- Visit homes that have the porch light on.
- Accept your treats at the door and never go into a stranger’s house.
- Use face paint rather than masks or things that will cover your eyes.
- Be cautious of animals and strangers.
- Have a grown-up inspect your treats before eating. And don’t eat candy if the package is already opened. Small, hard pieces of candy are a choking hazard for young children.
Guess Who Came to School Today?
October 18, 2007
Today, Mick Foley was our Principal for the Day! He visited all our classes, answered questions, signed autographs, and then took class pictures. The kids enjoyed every minute of it.
SLAM! Sports - Wrestling - Mick Foley
Welcome Back
September 10, 2007September is upon us and we’re all back in school.
I hope everyone was able to ‘chilax’!!!
Schedule for September
http://assignaday.4teachers.org/teachers/AssignmentManager.php?calendar_id=207583
