LIGHT YEAR
A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km. More p recisely, one light-year is equal to 9,500,000,000,000 kilometers.
Why would you want such a big unit of distance? Well, on Earth, a kilometer may be just fine. It is a few hundred kilometers from New York City to Washington, DC; it is a few thousand kilometers from California to Maine. In the universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance
ASTRONOMICAL UNIT
150 million kilometers/93 million miles
An Astronomical unit is the standard distance between the earth and the sun.
It is about 149,597,870.691 kilometers (92,955,807.267 miles).
It is defined as the distance that an object would have a perfectly circular orbit of exactly 365.2568983 days (31,558,196.01312 seconds).
PARSEC
A parsec is an astronomical unit of measurement that is equivalent to 3.26 light years distance, or the distance photons will travel in vacuum over the period of 3.26 years. Light travels at an approximate speed of 186,000 miles per second (300,000 kilometers per second), so a parsec represents a distance of just over 19 trillion miles (~31 trillion kilometers).
By comparison, the average distance to the Sun from Earth is only 93 million miles (150,000,000 km). This distance is referred to as one astronomical unit (AU). One would have to make 103,000 round trips to the Sun to cover the distance indicated by a single parsec (206,265 AUs = 1 parsec). Our solar system, defined for example by Pluto's orbit, is only 1/800ths of a light year across. It would have to be 2,608 times larger to equal 1 parsec across.
PARALLAX
Parallax causes adjacent pictures for a panorama to differ in ways that prevents them from being stitched together perfectly. It can cause ghosting, blurring, or even prevent stitching software from being able to work out where to position the pictures to be able to stitch them together.
It's really easy to see the effect of parallax: Hold up your index finger in front of you. Close one eye and line up your finger with something further away such as a door, piece of furniture, window, whatever. Now without moving your finger, rotate your head from left to right - your finger will seem to move slightly as you turn your head. Voilą! you are seeing parallax.
Exactly the same happens when you shoot pictures with your camera
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