How much of the Earth can you see at once?

If you are of average height, the horizon is a little less than 3 miles away, so you can see that far in a circle around you.

Call it 27 square miles give or take.

Earth’s surface area is, per Wikipedia, 197 millon square miles. So you can see a little less than 1/7,450,000th of the earth’s surface. Or just about 0.0000134% of it.

What is a light-year and how is it used?

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.

In our solar system, we tend to describe distances in terms of the Astronomical Unit (AU). The AU is defined as the average distance between the Earth and the Sun. It is approximately 150 million km (93 million miles). Mercury can be said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the Sun. The AU, however, is not big enough of a unit when we start talking about distances to objects outside our solar system.

For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the light-year or the parsec . The light-year we have already defined. The parsec is equal to 3.3 light-years. 

Using the light-year, we can say that :

• The Crab supernova remnant is about 4,000 light-years away.
• The Milky Way Galaxy is about 150,000 light-years across.
• The Andromeda Galaxy is 2.3 million light-years away.

Top 10 Things That Make Humans Special

Humans are unusual animals by any stretch of the imagination. Our special abilities, from big brains to opposable thumbs, have allowed us change our world dramatically and even leave the planet. There are also odd things about us that are, well, just special in relation to the rest of the animal kingdom. So what exactly makes us so special? Some things we take completely for granted might surprise you.

1. Speech

The larynx, or voice box, sits lower in the throat in humans than in chimps, one of several features that enable human speech. Human ancestors evolved a descended larynx roughly 350,000 years ago. We also possess a descended hyoid bone — this horseshoe-shaped bone below the tongue, unique in that it is not attached to any other bones in the body, allows us to articulate words when speaking.

2  Upright Posture

Humans are unique among the primates in how walking fully upright is our chief mode of locomotion. This frees our hands up for using tools. Unfortunately, the changes made in our pelvis for moving on two legs, in combination with babies with large brains, makes human childbirth unusually dangerous compared with the rest of the animal kingdom. A century ago, childbirth was a leading cause of death for women. The lumbar curve in the lower back, which helps us maintain our balance as we stand and walk, also leaves us vulnerable to lower back pain and strain.

3.      Nakedness

We look naked compared to our hairier ape cousins. Surprisingly, however, a square inch of human skin on average possesses as much hair-producing follicles as other primates, or more — humans often just have thinner, shorter, lighter hairs. Fun fact about hair: Even though we don't seem to have much, it apparently helps us detect parasites, according to one study.

4.      Clothing

Humans may be called "naked apes," but most of us wear clothing, a fact that makes us unique in the animal kingdom, save for the clothing we make for other animals. The development of clothing has even influenced the evolution of other species — the body louse, unlike all other kinds, clings to clothing, not hair.

5.      Extraordinary Brains

Without a doubt, the human trait that sets us apart the most from the animal kingdom is our extraordinary brain. Humans don't have the largest brains in the world — those belong to sperm whales. We don't even have the largest brains relative to body size — many birds have brains that make up more than 8 percent of their body weight, compared to only 2.5 percent for humans. Yet the human brain, weighing only about 3 pounds when fully grown, give us the ability to reason and think on our feet beyond the capabilities of the rest of the animal kingdom, and provided the works of Mozart, Einstein and many other geniuses.

6.      Hands

Contrary to popular misconceptions, humans are not the only animals to possess opposable thumbs — most primates do. (Unlike the rest of the great apes, we don't have opposable big toes on our feet.) What makes humans unique is how we can bring our thumbs all the way across the hand to our ring and little fingers. We can also flex the ring and little fingers toward the base of our thumb. This gives humans a powerful grip and exceptional dexterity to hold and manipulate tools with.

7.      Fire

The human ability to control fire would have brought a semblance of day to night, helping our ancestors to see in an otherwise dark world and keep nocturnal predators at bay. The warmth of the flames also helped people stay warm in cold weather, enabling us to live in cooler areas. And of course it gave us cooking, which some researchers suggest influenced human evolution — cooked foods are easier to chew and digest, perhaps contributing to human reductions in tooth and gut size.

8.      Blushing

Humans are the only species known to blush, a behavior Darwin called "the most peculiar and the most human of all expressions." It remains uncertain why people blush, involuntarily revealing our innermost emotions. The most common idea is that blushing helps keep people honest, benefiting the group as a whole.

9.      Long Childhoods

Humans must remain in the care of their parents for much longer than other living primates. The question then becomes why, when it might make more evolutionary sense to grow as fast as possible to have more offspring. The explanation may be our large brains, which presumably require a long time to grow and learn.

10.  Life after Children

Most animals reproduce until they die, but in humans, females can survive long after ceasing reproduction. This might be due to the social bonds seen in humans — in extended families, grandparents can help ensure the success of their families long after they themselves can have children.

Discovery and naming of Helium

Sir William Ramsay,
the discoverer of terrestrial helium

The first evidence of helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was initially assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D₃ Fraunhofer line because it was near the known D₁ and D₂ lines of sodium. He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, (helios). In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D₃ spectral line, when he analyzed a material that had been sublimated during a recent eruption of Mount Vesuvius.

On March 26, 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10% rare earth elements) with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line that matched the D₃ line observed in the spectrum of the Sun. These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite in the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight. Helium was also isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to nitrogen. His letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science.

What is Fraunhofer Spectram?

Fraunhofer Spectrum also called as Fraunhofer lines are a set of spectral lines named after the German physicist Joseph von Fraunhofer. The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.

In 1802, a scientist called W.H. Wollaston noticed that the visible spectrum from the Sun had several dark lines in it. Not long afterwards, Joseph von Fraunhofer built the first spectrometer. This focused sunlight from a small telescope onto a narrow slit. The light then passed through a prism, which produced the spectrum. Fraunhofer later invented the diffraction grating, which is used in most spectrometers today. Fraunhofer not only confirmed Wollaston's results, but also found that there were far more dark lines in the spectrum than Wollaston had suspected. Fraunhofer showed that these were a feature of sunlight and not an illusion nor an optical effect, and he labelled them with letters of the alphabet (A,B,C etc.). We now call these dark lines Fraunhofer lines.

What is Plato Telescope?

PLATO is a European Space Agency telescope also called as  Europe's Planet-Hunting Telescope. It is expected to launch in 2024. The name stands for "PLAnetary Transits and Oscillations of stars." The goal of this mission is to figure out under what conditions planets form and whether those conditions are favorable for life. 

To do this, PLATO will seek out and investigate Earth-size exoplanets, especially planets that orbit in the habitable zone around sun-like stars. (The habitable zone is usually defined as the area around a star where there is enough energy for liquid water on a planet's surface, although habitability also depends on other factors such as star variability.) It will determine how big their radii are; verify the mass of the planets from ground-based observatories; use astroseismology or "starquakes" to learn about a star's mass, radius and age; and identify bright targets for atmospheric spectroscopy along with other telescopes. If all goes to plan, the mission should be able to provide detailed information on hundreds of rocky and giant planets, providing more information about how solar systems form generally.

PLATO's primary mission is expected to last four years. However, the telescope is designed to last 6.5 years and its consumables, such as fuel, are expected to last about eight years. This means that the telescope could continue operations if its science mission was extended.

PLATO history

PLATO, which is named after the Ancient Greek philosopher Plato, was first proposed in 2007 after ESA put out a call for its Cosmic Vision 2015–2025 program. Cosmic Vision is the name of the current phase of ESA's long-term space science missions. 

ESA, like NASA, solicits opinions from the science community (ahead of selecting missions) to see what areas of space should be studied next. ESA then puts out calls for missions for a launch opportunity, attracting competitors that must present their science case.

PLATO was first proposed in 2007 as a part of Cosmic Visions, finishing assessment and definition phases in 2009 and 2010, respectively. ESA then put out a call in 2010 for a medium-class mission launch opportunity. 

PLATO, as well as two other missions — Solar Orbiter and Euclid (a mission to investigate dark energy and dark matter) —were selected as the finalists for this competition. Subsequently, Solar Orbiter was chosen for a 2017 launch date and Euclid for a 2020 launch date.

In February 2011, PLATO went up against four other medium-class mission candidates for a 2024 launch date. The others were EChO (the Exoplanet CHaracterization Observatory), LOFT (the Large Observatory For X-ray Timing), MarcoPolo-R (to collect and return a sample from a near-Earth asteroid) and STE-Quest (Space-Time Explorer and QUantum Equivalence principle Space Test). 

PLATO was selected in 2014 for the launch opportunity, which is also called M3 (for the third medium-class mission under Cosmic Visions.) The spacecraft is now in its design phase, which will take several years before it is finalized for construction.

PLATO science

The spacecraft will be launched from Earth on a Soyuz-Fregat rocket bound for a location called a Lagrange point. A Lagrange point is a relatively stable gravitational zone in space. PLATO will specifically be targeted for the L2 Lagrange point, a spot in space on the "dark" side of the Earth (meaning that the sun is always in the opposite direction.) 

L2 has been used before for the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck spacecraft, and is also the region where the James Webb Space Telescope will operate. Since L2 is relatively unstable, the spacecraft will follow a Lissajous orbit, which is a path around the Lagrange point, and periodically use fuel to stay in a consistent orbit.

The payload and science are contributed by a PLATO mission consortium (funded by European national funding agencies) while ESA provides the spacecraft, the CCDs, mission operations and part of the science operations.

PLATO's goal is to watch a large sample of bright stars for months or years, and measure them to high precision. By watching the stars for long periods, PLATO will be able to discern the light curve of the star, or the variations in its light transmitted over time. 

Since PLATO will last four years (at the least), the primary science mission will have it observe two regions of the sky for two years each. It's possible, however, that the telescope could instead do one long-duration observation of three years, and then move around in the sky for the fourth year of its primary science mission. 

"In view of the exceptionally fast development of exoplanet science, the final observing strategy will be investigated throughout the mission development and decided two years before launch," ESA said.

The long-term goal of many planetary observers is to find planets like Earth, and to seek signs of habitability on those other planets. While examining the atmospheres of these tiny planets will require a more advanced observatory, knowing where they are is a first step. 

Other space observatories looking for Earth-like planets include Kepler Space Telescope (in operation since 2009), the forthcoming Transiting Exoplanet Survey Satellite (TESS) and to a lesser extent, the forthcoming James Webb Space Telescope (JWST). Both TESS and JWST should launch in 2018.

PLATO instruments

PLATO has 24 normal cameras on board, arranged in four groups of six. Each of these groups has the same field of view, ESA said, but they are offset by a 9.2-degree-angle from the vertical axis of the spacecraft. Additionally, the spacecraft will have two "fast" cameras that will be used for brighter stars.

If an exoplanet passes in front of a star from the telescope's perspective, it can cause the light from the star to diminish, affecting its light curve. Other things can appear like planets, however, such as sunspots on the star that are darker than the surrounding surface and which also block the light. 

To verify any planets, PLATO will rely on backup observations from ground-based observatories. These observatories can measure the radial velocity of the star, or the velocity of the star along the line of sight from the observer. If slight tugs or movements are seen in the star, this would imply the presence of a planet due to the effect of the planet's gravity on the star.

"The key scientific requirement [is] to detect and characterize a large number of terrestrial planets around bright stars," ESA wrote in a statement. Terrestrial planets, being small, are tough to see around stars because they don't dim the star's light as much. The hope is that by observing closer and brighter stars, the planets will be a little larger and easier to spot.

What is evolution?

In biology, evolution is the change in the characteristics of a species over several generations and relies on the process of natural selection.

According to Hall & Hallgrímsson “Evolution is change in the heritable characteristics of biological populations over successive generations.”

• The theory of evolution is based on the idea that all species are related and gradually change over time.
  
• Evolution relies on there being genetic variation in a population which affects the physical characteristics (phenotype) of an organism.
• Some of these characteristics may give the individual an advantage over other individuals which they can then pass on to their offspring. 

What is natural selection?

• Charles Darwin’s theory of evolution states that evolution happens by natural selection.
• Individuals in a species show variation in physical characteristics. This variation is because of differences in their genes.
• Individuals with characteristics best suited to their environment are more likely to survive, finding food, avoiding predators and resisting disease. These individuals are more likely to reproduce and pass their genes on to their children.
• Individuals that are poorly adapted to their environment are less likely to survive and reproduce. Therefore their genes are less likely to be passed on to the next generation.
• As a consequence those individuals most suited to their environment survive and, given enough time, the species will gradually evolve.

Natural selection in action: the Peppered moth

• Before the industrial revolution in the mid-1700s, the peppered moth was most commonly a pale whitish color with black spots.
• This coloring enabled them to hide from potential predators on trees with pale-coloured bark, such as birch trees.
• The rarer dark-coloured peppered moths were easily seen against the pale bark of trees and therefore more easily seen by predators.
• As the Industrial Revolution reached its peak, the air in industrial areas became full of soot. This stained trees and buildings black.
• As a result, the lighter moths became much easier to spot than the darker ones, making them vulnerable to being eaten by birds.
• The darker moths were now camouflaged against the soot-stained trees and therefore less likely to be eaten.
• Over time this change in the environment led to the darker moths becoming more common and the pale moths rarer.

What have genes got to do with it?

• The mechanisms of evolution operate at the genomic level. Changes in DNA^? Sequences affect the composition and expression^? of our genes, the basic units of inheritance^?.
• To understand how different species have evolved we have to look at the DNA sequences in their genomes.
• Our evolutionary history is written into our genome. The human genome looks the way it does because of all the genetic changes that affected our ancestors.
• When DNA and genes in different species look very similar, this is usually taken as evidence of them sharing ancestors.
• For example, humans and the fruit fly, Drosophila melanogaster, share much of their DNA. 75 per cent of genes that cause diseases in humans are also found in the fruit fly.
• DNA accumulates changes over time. Some of these changes can be beneficial, and provide a selective advantage for an organism.
• Other changes may be harmful if they affect an important, everyday function. As a result some genes do not change much. They are said to be conserved.

Different types of evolution

Convergent evolution

• When the same adaptations evolve independently, under similar selection pressures.
• For example, flying insects, birds and bats have all evolved the ability to fly, but independently of each other.

Co-evolution

• When two species or groups of species have evolved alongside each other where one adapts to changes in the other.
• For example, flowering plants and pollinating insects such as bees.

Adaptive radiation

• When a species splits into a number of new forms when a change in the environment makes new resources available or creates new environmental challenges.

For example, finches on the Galapagos Islands have developed different shaped beaks to take advantage of the different kinds of food available on different islands.