What's 'green earth day' means?

Revolution of earth around sun

• QUESTION: I know that the orbit of the Earth around the Sun is an ellipes and that the Earth's axis is tilted, which means that either the Northern or Southern hemisphere is tilted toward the sun during perihelion, and the opposite hemisphere is tilted away. Which hemisphere is tilted toward the sun during perihelion? Also, since the orbit about the sun is an elipse, wouldn't that cause the hemisphere that is tilted toward the sun in perihelion, to have extremly hot summers, and cold winters; and the opposite hemisphere to have mild summers and mild winters since this hemisphere would be tilted toward the sun in anahelion? Or is the eliptical orbit of the Earth about the sun so close to a circular orbit that it is insignificant and causes extreme weather in both summer and winter? ANSWER: Hi Bryanna, Very good question, but the answer will surprise you. Perihelion is on or about January 3/4 at about 91.5 million miles where aphelion is on or about July 3/4 (the actual date depending on where you are in the leap year cycle) at about 94.5 million miles. So perihelion occurs very close to December 21 where the Earth's axis is pointing toward the sun for summer in the Southern Hemisphere. Turns out that the distance factor is only an additional 3-4% increase in solar radiation, the actual 23.5 degree tilt from the vertical accounts for the other 97% of seasonal changes. But since the Southern Hemisphere is more a "water" hemisphere than a land hemisphere (the real land hemisphere is the Northern Hemisphere)...and since water tends to moderate the surrounding land mass air temperatures, the water negates that 3-4% increase in air temperatures in the Southern Hemisphere due to our perihelion position. In July at aphelion, the 3-4 decrease is not even hardly measureable compared to the high amount of land masses in the Northern Hemisphere absorbing all that direct sunlight. So the high amount of land mass in the Northern Hemisphere offsets the 3-4% decrease in solar radiation loss due to aphelion, in the Northern Hemisphere. Of course, when the Earth's axis precesses half-way around it's 25,800 year precession cycle in 12,900 years (or 14,909 AD) then the situation is reversed, and only time will tell if the Northern Hemisphere will be a degree or two hotter in it's January summertime, and the Southern Hemisphere will be a degree or two cooler in it's July summertime. Unfortunately you and I won't be around to see, or record, that one. But basically, your last statement is the correct one, in that the near-circular orbit makes any "distance caused" temperature changes, insignificant compared to the seasonal changes caused by the 23.5 degree tilt from the vertical. Hope this helps, Clear Skies, Tom Whiting Erie, PA ---------- FOLLOW-UP ---------- QUESTION: What is the "Earth's axis precession cycle"? Is that where the axis is slowly changing its tilt angle over thousands of years time, causing the reversed situation you decribed in your answer? ANSWER: Hi Bryanna, Yes, the Earth's axis performs a little wobble, similar to what you see when a child's toy top is nearing the end of it's fast spin; the axis itself starts forming a little noticeable circle. In our case, it's the moon's influence, bulging out the earth a small amount, and not the "dying out" of our spin. The axial tilt (the 23.5 degree angle from the vertical) doesn't change; it remains pretty constant at 23.5 degrees from the vertical. But the axis itself, maintaining that tilt, performs a circle which we call precession. Currently our axis nearly points toward the 2nd magnitude star Polaris, well... 3/4 of a degree from Polaris. Our closest approach of our axis to Polaris is 1/2 degree in about 2105 AD. When the Pyramids were built, the star Thuban (Alpha Draconis) was their north star some 5000 years ago. In about 12000 years, the bright star Vega will be our North star. So it's a fairly big circle in the sky (a diameter of 47 degrees, or 23.5 doubled), projected outward, but at the geographic North Pole itself, the precession circle is only about the size of a tennis court. And naturally there are many centuries where their is no North Star at all....and of course the same is happening in the Southern Hemisphere too....right now there really is no South Star. But there will be someday....but not permanently as that side of our axis also has the same circle in the sky. It takes some 25,800 years for our axis to precess one entire orbit around the sky. So if your science book has the axis tilted to the right, in about 12900 years that same picture will picture the axis tilted to the left, and our Northern Hemisphere summer will be in the December/January calendar time frame, with our winters in June/July timeframe, and of course the reverse for the Southern Hemisphere. This is also why the "astrological signs" do not align with the astronomical constellations in the sky, as precession slowly changes the rising times of the constellations very slowly. With their mutual beginnings some 5000 years ago, astrologers decided to "ignore" precession, whereas astronomers could not ignore it as our calendars and view of sky would be incorrect. So this is why the "Astrological Signs" are now called "signs", and are about one full Zodiac Constellation in the sky...out of step with the real night sky. In fact, in some regions of the Zodiac, the signs are now TWO constellations out of step, and they get further and further apart as time goes by. (Of course, most (99.9%) us astronomers do not accept or believe in astrology anyway....or fortune telling, or alien visitations, flying saucers, zombies, ghosts, goblins, Ouiga boards, numerology, ... or any of that false/occult crap. But that's all another story. So that is what the axial precession is...no change in the tilt angle, just the tilt circular direction is slowly changing as time goes by, one trip around every approximately 25,800 years. Hope this helps, And please do rate my answers, Clear Skies, Tom Whiting Erie, PA PS...for confirmation of all this, just google Earth Axis Precession. tom FOLLOW UP: Opps...one minor error above....precession is caused by our equatorial bulge, (the equatorial diameter being slightly larger than the polar diameter) and not the moon's gravitational pull, as I stated above. If we were a perfect sphere, there would probably be no precession, or so small as to not be easily noticeable. Tom ---------- FOLLOW-UP ---------- QUESTION: Sorry to ask you so many questions, but it seems that everytime I read one of your answers another question pops in my head. You said that the North star changes. Is the north star the same star for everybody in the northern hemisphere, or does someone in Canada have a different north star than someone in Mexico? Because to my understanding the north star shouldn't move its position as the day goes by while the other stars appear to move across the sky( I know that the stars really don't move but appear to as the Earth rotates). That is because the star would be in the center so therefore appear to not move, but I would think that that stationary star should be a different star for people on different parts of the world, and not just in different hemispheres?

• Answer:

  Sorry, I guess I wasn't clear on that one. Of course Polaris is relatively "fixed" in our night sky for ANYONE in the Northern Hemisphere. What I meant was, over a period of thousands of years, as the Earth's axis precesses on that 25,800 year period, that's when where our polar axis slowly changes it's pointing direction... on the Celestial sphere. So it's not Polaris changing (the little dipper still looks like the Little dipper, with the North star at the tip of the handle)....it's where our axis is pointing...that's what slowly migrates around the northern sky. As I told you previously, Google Earth's Axis Precession, and it will give you diagrams and a sketch of where and when our axis direction, is pointing. Clear Skies, Tom FOLLOW UP: I see the best site to go to is athttp://en.wikipedia.org/wiki/Precession_(astronomy) Go about halfway down the page to "changing pole stars", and punch on the top star sketch to enlarge it (If you don't enlarge it, the AD dates look all screwed up). Notice at 3000 BC Thuban in Draco was our north star, around 2000 AD (now) Polaris is, and around 4000 AD the star Gamma Cephei will become our north star, as the circle describes our axial precession position in space. Gamma in 4000 AD will not be quite as close as Polaris is now, but we have no control over it, and that's the best that nature will give us for a while. Notice that there's a fairly long period of over 1000 years where there will be no north star for the N. Hemisphere, from about 2500 - 3500 AD, as our axis points between Polaris and Gamma Cephei. Clear Skies, Tom FOLLOW UP ____-----Opps, sorry, wrong website, it's athttp://en.wikipedia.org/wiki/Precession_(astronomy)Oh, Oh...it won't come up because it deletes the final word "astronomy", so you'll have to Google it (Earth axis precession) yourself, and punch on the Wikipedia URL that says at the end, "astronomy"....sorry for the inconvience, but I can't control the URL choosings. Tom