An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The fraction of time that a → pulsar undergoes → pulse nulling. For most → nulling pulsars this fraction can range from zero (for the → Vela pulsar) to more than 50%.

→ null; → -ing; → fraction.

The conversion of a → number system with a given → base to another system with a different base; such as the conversion of a → decimal number (base 10) to a → binary number system (base 2). In order to convert a number into its representation in a different number base, we have to express the number in terms of powers of the other base. For example, to convert the decimal number 100 to base 3, we must figure out how to express 100 as the sum of powers of 3. We proceed as follows:
1: Divide the decimal number to be converted (100) by the value of the new base (3).
2: Get the remainder from Step 1 (that is 1) as the rightmost digit (least significant digit) of new base number.
3: Divide the quotient of the previous divide (33) by the new base.
4: Record the remainder from Step 3 (0) as the next digit (to the left) of the new base number.
Repeat Steps 3 and 4, getting remainders from right to left, until the quotient becomes zero in Step 3 (2 and 0).
The last remainder thus obtained (1) will be the most significant digit of the new base number.
Therefore, 100₁₀ = 10201₃.
Conversely, to convert from another base to decimal we must:
1: Determine the column (positional) value of each digit.
2: Multiply the obtained column values (in Step 1) by the digits in the corresponding columns.
3: Sum the products calculated in Step 2. The total is the equivalent value in decimal.
For example, the binary number 1100100 is determined by computing the place value of each of the digits of the number:
(1 × 2⁶) + (1 × 2⁵) + (0 × 2⁴) + (0 × 2³) + (1 × 2²) + (0 × 2¹) + (0 × 2⁰) = 64 + 32 + 0 + 0 + 4 + 0 + 0 = 100.

→ number; → system; → conversion.

Another name for → numerical modeling.

→ numerical; → simulation.

1) Mechanics: A wobbling motion of a spinning → rigid body, such as a top, as it → precesses about its vertical axis.
2) Astro.: A slight nodding motion of the Earth's axis of rotation, which has a principal period of 18.6 years. It is primarily caused by lunar → perturbations, and is superimposed on the → precession of the equinoxes and moves the equinox as much as 17'' ahead of or behind its mean position.

Fromm L. nutation-, from nutat(us), p.p. of nutare "to wobble, to sway, to nod repeatedly," from nu "nod" + -ta frequentative suffix + -tus p.p. ending + -ion a suffix denoting action or condition.

Kaltâv, from Kermâni keletow, Malâyeri kallatow "wobbling," from kal, kalleh "head" + tâv, tow, tâb "swing, twist," from tâbidan "to twist, to spin."

Same as → equation of the equinoxes.

→ nutation; → right ascension.

1) The act or process of nourishing or of being nourished.
2) The science or study of, or a course of study in, nutrition, especially of humans (Dictionary.com).

Verbal noun from L. nutrire, → nourish.

A person who is trained or expert in the science of nutrition.

→ nutrition; → -ist.

The minimum number of resolution elements required to properly sample a signal, such as a star image, without causing erroneous effects known as aliasing. For electronic imaging, this number is generally taken as 2 pixels across the seeing disk diameter at the half intensity points. Also called → Shannon's sampling theorem and → sampling theorem.

Named after Harry Nyquist (1889-1976), a Swedish-born American physicist, who made important contributions to information theory, and Claude Elwood Shannon (1916-2001), an American mathematician and pioneer of information theory; → theorem.

A loosely bound grouping of O and B stars that typically stretches up to several hundred → light-years and may contain between a dozen and several hundred → O stars and → B stars. The members of an OB association are young and of roughly the same age. OB associations dissipate in a few tens of millions of years.

O and B, from spectral types; → association.

The outermost of Uranus' large satellites and the second largest. It has a diameter of 523 km and orbits 583,420 km from its planet. Compared to Uranus' moons Ariel, Titania, and Miranda, Oberon is heavily cratered. Like all of Uranus' large moons, Oberon is composed of roughly half ice and half rock. Oberon was discovered by Herschel in 1787.

Oberon is the King of the Fairies and husband of Titania in Shakespeare's Midsummer-Night's Dream.

1) The act of objecting.
2) A statement presented in opposition.

Verbal noun of → object.

1) Something by which a person is bound or obliged to do certain things, and which arises out of a sense of duty or results from custom, law, etc.
2) Something that is done or is to be done for such reasons (Dictionary.com).

M.E. obligacioun, from O.Fr. obligacion "obligation, duty, responsibility," from L. obligationem "an engaging or pledging," literally "a binding," noun of action from p.p. stem of obligare "to bind, bind up, bandage," from → ob- "to" + ligare "to bind," from PIE root *leig- "to bind."

Fariz, from Mid.Pers. frêz "obligation; duty."

Elongated, usually from the square or circular form.

Late M.E. oblonge, from L. oblongus "rather long," from ob- a prefix meaning "toward, to, on, over, against" + longus, → long.

Derâzak, from derâz "long," → length + -ak suffix.

1) The act of obscuring.
2) The state of being obscured.

→ obscure; → -tion.

1) Act or instance of observing; → observe.
2) Careful watching of an astronomical object or event using appropriate instruments, usually for collecting data.

Verbal noun of → observe.

Pertaining to, or founded on observation, especially based on observation rather than theory.

Adj. of → observation.

That part of astrophysics that is mainly concerned with the collection of observational data, in comparison with theoretical astrophysics

→ observational; → astrophysics.

An error in observation arising from systematically favoring brighter or weaker objects or some particular object morphologies; e.g. → Malmquist bias.

→ observational; → bias.

The application of observational data to the study of the Universe as a whole.

→ observational; → cosmology.

A feature appearing in an observation, which is not intrinsic to the object observed, but is due to the inappropriate method used (e.g. limited angular resolution).

→ observational; → effect.