Celestial Motion The Hubble Ultra-Deep Field is a longexposure photograph of a small part of the sky taken over the course of five months by the Hubble Space Telescope. The area of the sky photographed is about 3 arcminutes across. This is just one-tenth of the diameter of the full Moon as viewed from Earth, smaller than a 1 mm by 1 mm square of paper held at 1 meter away, and equal to roughly one thirteen-millionth of the total area of the sky. The majority of the lights in this photo are entire galaxies, each containing billions of stars. There are about ten thousand galaxies in the Hubble Ultra-Deep Field. This is arguably one of the most important and humbling photographs ever taken. MOVEMENT IN THE COSMOS THE NATURE OF GRAVITY MARS THE MOON URANUS NEPTUNE EARTH SATURN VENUS JUPITER MERCURY F=G ( ) m1 x m2 r2 F is the force from gravity. G is the gravitational constant. m1 and m2 are the masses of the bodies. r is the distance between their centers. NEWTONS FIRST LAW OF MOTION WHAT IS GRAVITY? SPEED IS RELATIVE GRAVITATIONAL WELLS . MEASURING GRAVITY WITH ISAAC NEWTON’S .LAW OF UNIVERSAL GRAVITATION Isaac Newton’s first law of motion states that an object in motion will stay in motion unless acted upon by an outside force. This also applies to objects at rest. Objects in space move because a force acted on them, or is acting on them. This is one of the core principles of physics and is vital to understanding motion. Gravity is a force that is responsible for a large portion of motion in the universe. The gravitational force is one of the four fundamental forces of physics, and acts by pulling all matter together. Gravity is actually the weakest of the four forces. The reason that we notice it so much is because gravity works over long distances and is always attractive. The more mass an object has, the larger the gravitational force it exerts on other objects. Another important concept to understand is that we have no way to measure absolute speed. Speed is completely relative to us. Humans can only detect acceleration, and can only measure speed relative to something else. Every object has a gravitational well. This well is a conceptual model of the gravitational field that surrounds a body of matter. The more massive the body, the deeper and more extensive the well. Things on the surface of the body are at the bottom of the well, and as they leave the body’s atmosphere and escape its gravitational field, they climb out of the well. The deeper the well, the more energy it takes to escape. Isaac Newton was the first to accurately describe an equation that could calculate the force from gravity. The gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. This equation is called Newton’s law of universal gravitation. This law has since been superceded by Einstein’s theory of general relativity, but it is still used as an excellent approximation of the effects of gravity. Relativity is only necessary when dealing with extremely massive and dense objects, such as black holes, or when there is a need for extreme precision. THE NATURE OF ORBITS THE SHAPE OF ORBITS Orbits vary in their shape. They can be elliptical, or almost perfectly circular. The moon’s orbit of the earth is slightly elliptical. The earth is at one foci of this orbital ellipse, while the other foci is empty. balanced orbit low velocity high velocity PERPETUAL FREEFALL EQUAL AREAS IN EQUAL TIMES We all know that the moon orbits the sun, but what a lot of people seem to misunderstand is the nature of an orbit. When an object is orbiting around another object, that object is in freefall. The closer an object is to whatever it’s orbiting, the faster it will travel. Johannes Kepler observed an interesting relationship in orbital physics, which is known as the Second Law of Kepler. It states that an object, following its orbital path, sweeps out equal areas in equal times. However, it has a velocity perpendicular to the object that it is orbiting. At the right velocity, this means that the object will essentially stay in freefall indefinitely. If the velocity is too high, the orbiting object will break out of the orbit and fly off into space. Too low, and the object will crash into whatever it’s orbiting. The orbiting object travels faster when it is closer to a source of gravity, so both blue lines in the diagram took the object equal times to travel across, even though they are of different length. The second law of Kepler states that the grey wedges drawn by those lines are of equal areas. This is what is meant by “equal areas in equal times.” mass diameter 1.90x1027 kg 139822 km mass diameter 5.69x1026 kg 116464 km mass diameter mass diameter MERCURY 3.30x1023 kg 4879 km mass diameter 4.87x1024 kg 12104 km VENUS mass diameter EARTH 5.98x1024 kg 12742 km mass diameter MARS JUPITER There are eight planets in our solar system. The four closest to the sun are terrestrial planets (rocky planets), and the four farthest form the sun are jovian gas giants. This is because the gas from which they were formed could only condense into harder elements under more heat. All of the planets orbit the sun in the same plane and in the same direction. This is due to how solar systems are formed. mass diameter 6.42x1023 kg 6779 km URANUS OUR NEIGHBORS 8.68x1025 kg 50724 km SATURN NEPTUNE 1.02x1026 kg 49244 km SOLAR SYSTEM FORMATION Tiny particles around the star began condensing from the gas. Collisions between these particles let them grow and attract even more particles. These were the seeds of our planets. A cloud of interstellar gas becomes slightly too dense and very slowly begins contracting because of its own gravity. It gets more dense and contracts faster. The contraction is non-uniform, so the cloud starts spinning. Eventually, the clouds of gas at the center of the disc were heated to 10 million Kelvin. Nuclear fusion began and our sun was born as a star. The disk stops contracting due to reaching gravitational equilibrium and begins to lose heat in the form of infrared radiation. The spinning cause the cloud to take on a disk-like shape. Most of the matter is concentrated near the center of the disk. Colliding particles from the contraction generate heat. THE MILKY WAY The milky way galaxy - our home galaxy - is a barred spiral galaxy. It has two arms that extend and reach around its center. Near the middle, there is a bar-shaped structure that is composed of stars. The milky way is 100,000-200,000 light years in diameter and contains 200-400 billion stars. Many of these stars have planets of their own. These are incomprehensibly large numbers. Some analogies help put them into frame. If the sun were the size of a grapefruit, the earth would be the size of the head of a pin, and they would be 16 meters apart. Pluto would be 539 meters away from the sun. If the whole solar system out to pluto was the size of a US quarter, our milky way galaxy would be about the size of the United States of America. Our solar system is about two-thirds of the way out from the galactic center, on the inner edge of a spiral. It takes about 200 million years for the sun to complete a revolution around the center of the galaxy. The galaxy as a whole is traveling as well. Galaxies are formed in much the same way as solar systems, just on a much, much larger scale. Instead of forming a star at the center, there is enough mass and gravity to form a black hole. Our solar system is just one star among millions in one galaxy among millions more, and it’s all plummenting through space. Typefaces used: Univers LT 45 Light Univers LT 65 Bold Univers LT 59 Ultra Condensed Garamond Premiere Pro Semibold Italic Sources: wikipedia.org nasa.gov universtoday.com Illustrated and written by Boris Boguslavsky