Archive | January, 2016

What is a parallax? D.2.1

31 Jan

Hold your finger out in front of you, as you do this focus your look at some distant object.

Spot something like a distant tree, a mountain, a boat out in the ocean, whatever you choose.

Hold your finger out and then close one eye. Hold it with your left and now switch from your left eye to your right eye. Did you notice your finger jumped!

Go back and forth, and you will watch your finger jump back and forth as you shift viewing positions between left and right.

That’s parallax and this relates to viewing depth perception particularly when we look at the universe

Sun-centered solar system D.1.31

30 Jan

The idea that the Earth would first go around the Sun instead of the Earth at the center was proposed in 260 BCE by the Greek astronomer Aristarchus – Except his contemporaries rejected the idea.

A Sun-centered solar system did not gain eventual acceptance until some 1700 years later during the renaissance.

There were many reasons why the Greeks were reluctant to abandon an Earth-centered universe and there were philosophical and psychological reasons associated with that as well. The power of the church played a large part in this.

The most important physical reason was that their inability to detect something known as parallax. We know it parallaxes. Our brains intrinsically know what parallax is.

That’s how we get depth perception.

The Epoch we live in D.1.30

29 Jan

The solar eclipses happen for a very unusual reason and it’s only true in the epoch we happen to live in.

The Sun is physically about 400 times bigger than the moon.

The epoch we’re living in, the Sun is about 400 times farther away. And so this complete coincidence that 400 times bigger, but 400 times farther away gives the Moon and the Sun about the same size, and so you get the possibility of having solar eclipses.

That wasn’t true a few billion years ago, when the Moon was a lot closer, and it’s not going to be true in a few billion years when the Moon is a lot farther away.

So solar eclipses are a special treat for the epoch we happen to be living in.


A Solar Eclipse D.1.29

28 Jan

A solar eclipse occurs when the Moon is between the Sun and the Earth. And it’s the Moon’s shadow that goes onto the Earth. There is a full moon once a month and that also means a lunar eclipse.

There is a new moon every month and that is a candidate for a solar eclipse. There isn’t an eclipse every month and that’s because the moon’s orbital plane is inclined by about five degrees relative to the ecliptic, which is the plane in which the planets orbit.

The moon is inclined, relative that orbit plane by about five degrees. Most of the time the moon, when it’s in its full moon phase, is above the Earth’s shadow, and alternatively in the new moon phase the moon is below the line connecting the Sun and the Earth. And so because of this difference we don’t see either a lunar eclipse or a solar eclipse every month, it only happens, generally a couple of times a year when the conditions are favourable, such that the moon is exactly in the line of the ecliptic, and we can then either get a lunar eclipse or a solar eclipse.

The solar eclipses happen for a very unusual reason and it’s only true in the epoch we happen to live in.

The Sun is physically about 400 times bigger than the moon.

The Eclipses D.1.28

27 Jan

There is the full moon when the orientation is the sun, the Earth, and the Moon. There are the stages of the moon from the many phases to the new moon.

There is unison with the Sun, the Moon, and the Earth.

Both the moon and the Earth cast shadows, and these shadows can create eclipses when the Sun, the Earth, and the Moon are in a single line.

A lunar eclipse occurs when the earth lies between the sun and the moon. And the Earth’s shadow then goes onto the Moon.

Moon Time D.1.27

26 Jan

The sun has always been known to indicate the time of day but there is another time piece – The Moon.

The moon can tell us the time. The moon was used as a clock in antiquity and people used it to tell the time before the use of clocks.

By observing the phase of the moon and where it is in the sky, whether it’s just on the horizon or directly over head or in the set phase. You can tell the time usually pretty good to within about an hour or so.

So that’s how we can tell the time using the moon.

The moon orbits the sun, in fact all the planets orbits the sun. And there is a spiral pattern as both the Earth and the Moon orbit each other; they orbit around a common center of mass. And that common center of mass orbits around the sun.

The sun and the moon shed a time system.

The Moon D.1.26

25 Jan

As the moon moves through the sky, its appearance, its rise time, and its set time change with the cycle of the lunar phases.

The moon takes 27 days to complete an orbit, and the time that it takes to go through a full cycle of phases is about 29.3 days, and the difference between it’s orbital period, 27.3 and the time that we go through the phases, about 29 and a half days, is due because the Earth is moving in its orbit.

Over the course of the month, the moon moves in its orbit. And the moon has to go a little bit farther in order to catch up with the Earth and hence, that’s why the orbital period and the cycle of the phases is not quite the same.

And, the root word of month is Moon, and that’s the origin of it.

Solstices and equinoxes D.1.25

24 Jan

The constellations are associated with solstices and equinoxes.

About 3,000 years ago, or around when the Babylonians were active, the Sun appeared in the constellation of Cancer at the summer solstice, but it now appears in Gemini.

And you can see the impact of precession on any world map because it’s marked as the latitude at which the sun was directly overhead, 3,000 years ago, at 23 degrees north, that’s why it’s called the Tropic of Cancer.

Similarly, the southern one is the Tropic of Capricorn, because on the solstices on those days, the sun would be directly overhead. But the Sun is no longer directly overhead in Cancer because of precession, it’s now in Gemini.

When astrology began about 3,000 or so years ago, your sign was the constellation in which the sun appeared on your date.

Polaris and Vega Stars D.1.24

23 Jan

The earth’s rotation wobbles just like a top does. This happens on a very long time scale. This is not a yearly occurrence but instead this cycle takes about 26,000 years to complete a full cycle.

Currently, the earth’s rotation access points toward the star Polaris. The star Polaris is the marker for going north at night.

In 13,000 years when it’s halfway through its precession cycle, the Earth’s rotation pull is going to be pointed more in the direction of the star of Vega.

So Polaris will no longer be the north beacon that it is now. And then in another 13,000 years later this earth’s top will wobble around and we will be back to Polaris on 26,000 year time scales.

So the precession as it wobbles around does not change the axis tilt, it’s always 23.5 degrees.

The Seasons D.1.23

22 Jan

The seasons are caused by the 23.5 degree tilt of Earth’s rotation axis. The celestial sphere is tilted by 23.5 degrees.

The rotation pole which is tilted 23 degrees relative to the north ecliptic pole, which would then be perpendicular to the ecliptic pole.

There is this 23.5 degree angle between the orbital plane and the Celestial Equator.

And as Earth rotates, or orbits, the Sun, that 23.5 degree angle always remains constant.