Tides are distortions that occur in the shape of a celestial body. They are caused by the gravitational force of one or more other celestial bodies on that first body.


Time is a measurement to determine the duration of an event or to indicate when an event occurred. For example, one could say that it took an object 3.58 seconds to fall, indicating how long it took for that event to occur.


Topology is a branch of mathematics sometimes known as rubber-sheet geometry. It deals with the properties of a geometric figure that do not change when the shape is twisted, stretched, or squeezed.


A tornado is a rapidly spinning column of air formed in severe thunderstorms. The rotating column, or vortex, forms inside the storm cloud (cumulonimbus), then grows downward until it touches the ground.


Touch is one of the five senses through which animals interpret the world around them. (The other senses are smell, taste, sight, and hearing.) While the organs of the other senses are located primarily in a single area (such as sight in the eyes and taste in the tongue), the sensation of touch can be experienced anywhere on the body, from the top of the head to the tips of the toes.


A tranquilizer is a drug that acts on the central nervous system and is used to calm, decrease anxiety, or help a person to sleep. Often called depressants because they suppress the central nervous system and slow the body down, they are used to treat mental illness as well as common anxiety and sleeplessness.


A transformer is an electrical device used to change the voltage in an electric circuit. It usually consists of a soft iron core with a rectangular shape.


A transistor is a solid-state electronic device used to control the flow of an electric current. The term solid-state refers to devices that take advantage of special properties of solids.

Transition Elements

The transition elements are the elements that make up Groups 3 through 12 of the periodic table. These elements, all of which are metals, include some of the best-known names on the periodic table—iron, gold, silver, copper, mercury, zinc, nickel, chromium, and platinum among them.

Transplant, Surgical

A surgical transplant involves removing organs or tissues from one person and replacing them with corresponding ones from another part of that person's body or from another person. The idea of surgical transplantation dates back several centuries, but it has become a practical medical approach only in the last few decades of the twentieth century.


A tree is a woody perennial plant that has a single trunk arising from the ground (typically without branches near the base) and that usually grows to 20 feet (6 meters) or more in height. Branches and twigs grow from the trunk of a tree to form its characteristic leafy crown.


Trigonometry is a branch of mathematics concerned with the relationship between angles and their sides and the calculations based on them. First developed during the third century B.C.


A tumor is an abnormal growth of tissue. Also known as a neoplasm (meaning "new formation"), a tumor can be either benign (not serious or harmful) or malignant (cancerous or deadly).


Tunneling is a phenomenon in which a tiny particle penetrates an energy barrier that it could not, according to the classical laws of science, pass across. One way of describing this process, also known as the tunnel effect, is shown in Figure 1.


The term ultrasonics applies to sound waves that vibrate at a frequency higher than the frequency that can be heard by the human ear (or higher than about 20,000 hertz).

Ultraviolet Astronomy

Matter in the universe emits radiation (energy in the form of subatomic particles or waves) from all parts of the electromagnetic spectrum. The electromagnetic spectrum is the range of wavelengths produced by the interaction of electricity and magnetism.


In geology, uniformitarianism is the belief that Earth's physical structure is the result of currently existing forces that have operated uniformly (in the same way) since Earth formed roughly 4.5 billion years ago. Mountains rise, valleys deepen, and sand grains collect now the same way they uplifted, eroded, and deposited over these millions of years.

Units and Standards

A unit of measurement is some specific quantity that has been chosen as the standard against which other measurements of the same kind are made. For example, the meter is the unit of measurement for length in the metric system.


Uranus, the seventh planet from the Sun, was probably struck by a large object at some point in its history. The collision knocked the planet sideways, giving it a most unique orbit.


A vaccine is a substance made of weakened or killed disease germs designed to make a body immune to (safe against) that particular infectious disease. Effective vaccines change the immune system (the body's natural defense system against disease and infection) so it acts as if it has already developed a disease.


The term vacuum has two different meanings. In its strictest sense, a vacuum is a region of space completely lacking any form of matter.

Vacuum Tube

A vacuum tube is a hollow glass cylinder from which as much air as possible has been removed. The cylinder also contains two metal electrodes: the cathode, or negative electrode, and the anode, or positive electrode.

Variable Star

Variable stars are stars that vary in brightness over time. In most cases, these changes occur very slowly over a period of months or even years.


Venus, the second planet from the Sun, is the closest planet to Earth. It is visible in the sky either three hours after sunset or three hours before sunrise, depending on the season.


Vertebrates are any animals that have a backbone or spinal column. These animals are so named because nearly all adults have vertebrae, bone or segments of cartilage forming the spinal column.

Video Recording

Video recording is the process by which visual images are recorded on some form of magnetic recording device such as tape or a video disc. In magnetic recording, an unrecorded tape is wrapped around a rotating drum that carries the tape through a series of steps before it leaves as a recorded tape.

Virtual Reality

Virtual reality is an artificial environment that is created and maintained by a computer and that is at least partly shaped and determined by the user. A virtual reality system allows the user to "leave" the real world and step into a world whose sensory inputs (sights, sounds, smells, etc.) are provided not by natural objects but by computer-created means.


A virus is a small, infectious agent that is made up of a core of genetic material surrounded by a shell of protein. The genetic material (which is responsible for carrying forward hereditary traits from parent cells to offspring) may be either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).