Welcome to SkyTours with Derrick! If you've ever found yourself under the night sky wondering what that thing is, well, you've come to the right place to find out. I'll provide regular postings of just what's available for you to see at this time of this year, including planets, stars, constellations and my favorite - satellites! I'll also welcome your suggestions for what to add to the blog for your information and answer your questions.

Monday, July 17, 2017

2017 Super Solar Eclipse, Pt. 3 - Einstein and the 1919 Principe Solar Eclipse

In 1915, physicist Albert Einstein proposed a new, more comprehensive understanding of how the universe works. He called his new idea the theory of General Relativity. Einstein’s newest theory suggested that space and time are knitted together throughout the universe as a sort of invisible ‘fabric’ that is distorted - dimpled- by massive objects. Einstein named the fabric ‘space-time’ and re-defined gravity as how we feel and interpret the distortion of space-time caused by massive objects. The motion of the planets around the Sun, for example, is caused by the Sun’s mass distorting the surrounding space-time field into the shape of a well.

Try this at home: four people each grab a bed sheet at the corners and pull until the sheet is tightly stretched. Now a fifth person places a heavy ball (at least a basketball, but lighter than a bowling ball) on the sheet. As the ball rolls to the middle of the sheet it distorts the surface as it moves. That distortion of the sheet as it rolls is much like how space-time is distorted by massive objects, like stars. Use a smaller ball to try making an orbit around the bigger ball. The motion of the smaller ball is determined by the distortion caused by the bigger ball. It’s the distortion of space-time in a bed sheet!

But what does Einstein’s theory of General Relativity have to do with eclipses?

In 1919, British astronomer Arthur Eddington understood that the curvature of space-time should also bend light rays, so he figured that a good test of the theory would be to use the curved space-time field around the Sun to bend light from a star cluster positioned just behind the sun. To see that cluster just next to the sun, the adjacent sky would need to be dark. Eddington used the darkened sky of the May 29, 1919 total solar eclipse at the island of Principe, just off the coast of West Africa, to see the cluster. His measurements of the cluster’s position were close enough to Einstein’s predictions to confirm the theory of General Relativity and catapult Einstein to international stardom. Since then, tests of Einstein’s predictions carried out during total solar eclipses have continually confirmed the validity of his theory and existence of the space-time continuum.

Next Time: Where to go and how to view the August 21 eclipse!

2017 Super Solar Eclipse, Pt. 2 - Why Don't Eclipses Happen Every Month?

As we said in our first installment in this series, solar eclipses are about light and shadows. As the moon passes between the Sun and the Earth, the moon blocks light from the Sun creating a shadow that falls onto the Earth. Seems simple enough, doesn't it? But how can our little moon block all the light from our gargantuan sun? And why doesn't this happen every month?

First, the size. The sun is 400 times farther away from us than the moon is. The sun is also 400 times bigger than the moon. This situation creates a 1 to 1 ratio of size over distance. Viewed from Earth, the two objects now appear to be the same size. So when the moon passes directly between the sun and a point on Earth, the moon appears to completely cover the sun and the sun is said to be eclipsed by the moon. If the moon were smaller or even farther away, there would be no total solar eclipses. In fact, the moon is slowly orbiting away from Earth. At some point in the earth’s future, total solar eclipses won’t happen anymore.

(Try this at home: Use a soccer ball for the sun and a tennis ball for the moon. Now hold the tennis ball directly in front of the soccer ball and adjust the tennis ball’s distance from the soccer ball so that the tennis ball seems to just match the soccer ball’s size. This is exactly what’s happening between the sun and the moon as seen by an observer on Earth!)

Second, why not every month? If the orbits of the Earth and the moon were on the same plane, there would be solar and lunar eclipses every month. But the moon’s orbit has a slight tilt - about 5 degrees from horizontal. This means that only two points of the moon’s orbit line up directly with the Earth’s orbit every month. If the moon’s orbit weren’t tilted, we’d have a solar eclipse every month! The tilt of the orbit, the rotation of the moon’s orbit and the Earth-Moon system’s revolution around the sun, limit the number of solar eclipses to just two or three per year. Add in a few other orbital variations and solar eclipses in specific locations become very rare. The last total solar eclipse in Philadelphia: 29 Jul 1478. The next? 01 May 2079!

Next time, Einstein and the 1919 Solar Eclipse!

2017 Super Solar Eclipse, Pt. 1 - What's An Eclipse?

It’s coming. August 21st. The slowly progressing cycles of Earth and Moon orbits around the sun brings us closer to a special triple alignment every minute. A dedicated group of observers who literally ‘chase’ these special alignments across our planet, wait patiently but plan thourougly their next expedition to bask in… the dark of the moon. This special alignment, called a solar eclipse, is visible from someplace on Earth about every 18 months -  that’s two total eclipses every three years. Often described as the most spectacular astronomical event to be seen from Earth, I recommend that no human should leave the planet without seeing a solar eclipse. Mabel Loomis Todd, a close friend of Emily Dickenson and an avid eclipse chaser in the 19th century, said this after witnessing the May 28, 1900 total solar eclipse at Tripoli, North Africa: 

“I doubt if the effect of witnessing a total eclipse ever quite passes away. The impression is singularly vivid and quieting for days, and can never be wholly lost. A startling nearness to the gigantic forces of nature and their inconceivable operation seems to have been established. Personalities, towns, and cities, and hates and jealousies, and even mundane hopes grow very small and far away.”

Your next chance to see one? August 21st. Don’t miss it!

There are three types of solar eclipse visible from Earth; total, partial, and annular. While the first two types seem pretty straight-forward, that last one might be a bit challenging to understand. But before we figure that one out, let’s start at the beginning: What is a solar eclipse?

A solar eclipse is an astronomical event observed on Earth when light from the sun is blocked by the moon. Eclipses are all about the alignment of these three objects, which one is blocking the sun’s light, and the shadows they create. When the moon lines up directly between the Sun and the Earth, it blocks sunlight from reaching a narrow strip of Earth for a few hours. It’s a narrow strip and there’s a time element involved because the earth and the moon are in constant motion. This event is a total solar eclipse; the sun is ‘eclipsed’ - completely blocked - by the moon. If the moon doesn’t exactly line up in front of the sun, only part of the sun is eclipsed and the event is a partial solar eclipse. Then there’s the very special case – the annular eclipse, where the moon lines up directly in front of the sun but isn’t big enough to completely cover the sun. In this case a ring of sunlight encircles the moon as it stands in front of the sun.

Sound complicated? They can be but for the casual observer, it’s just light and shadows. Try this experiment: Stand outside in bright sunlight. Allow your shadow to fall on a nearby person, plant or pet. Your body, blocking light from the sun, creates a shadow, causing a solar eclipse for whatever is in your shadow. Exactly the same as the moon; blocking light from the sun, it creates a shadow, causing a solar eclipse for whatever is in its’ shadow. Get it? That’s all an eclipse is! For scientists who study eclipses, they are incredibly complicated and each solar eclipse has its own signature.

In the next installment, eclipse mechanics and why they don’t happen every month!

This five-part series will look at why eclipses don’t happen every month, a brief history of observing eclipses, how Einstein used an eclipse to prove his theory of general relativity, and of course, how you can become an eclipse observing expert just by holding a spaghetti strainer!