© J R Stockton, ≥ 2009-08-13

Links within this site :-

**Merlyn Home Page**- Site Index, E-Mail, Copying- My other Astronomy / Astronautics pages :-
**This Page**- JavaScript Calculations :-- Functions Commonly Used on This Page
- Depression of Horizon from Altitude
- Elevation of Object from Position
- Mapping between an Orb and a Circle
- Date Arithmetic
- Time Conversion
- TLE, JD, MJD, CJD & CMJD
- Adding and Subtracting Stellar Magnitudes
- Travel Calculations :-
- Distance from Acceleration and Time - Newtonian
- Time for Distance at Acceleration - Newtonian & Relativistic

- Parallax and Resolution

- JavaScript Index and Introduction
- JavaScript Include Files

For a general JavaScript calculator, in different versions, see
Gravity 0,
JavaScript/HTML/VBS Quick Trials,
JavaScript Demos.

**N.B.** Developed in MSIE 4/6; now maintained in Firefox 3.0;
occasionally checked in some other browsers.

General JavaScript functions are coded in include1.js and include3.js, and are shown in JavaScript Include Files.

Most values entered can now have spaces, e.g. as thousands
separators; `eval()` is used in `userIn()`. Inputs
can be expressions such as `29028*0.3048`. Comment is
allowed.

Some of these assume a spherical body. For Earth, use a circumference of 40,000 km or better (that matches the originally-intended size of the metre).

See JavaScript Date and Time Introduction
*ff.* for details,
and in Leap Seconds.

Note the scroll bar.

On Earth, the depression in degrees is close to the square root of the height in kilometres.

Use consistent length units; approximate Earth size in kilometres is preloaded.

The initial value of Altitude is roughly correct for ISS; the Viewing Distance for the Glasgow-London latitude difference.

Drawing a matching diagram is left as an exercise for the reader.

The Viewing Distance calculation is iterative.

A distant Orb is viewed, from directly above Longitude 0 on its Equator; its North is at the top. Its surface is seen as a circle, in which one can use centred Cartesian (X, Y) or Polar (r, θ) co-ordinates.

Gregorian. Press any button to calculate its line from the other two entries. Not d/m/y; year>99.

Use Days ± = 0 to check.

For UNIX *time_t* to/from GMT, see in
JavaScript Date and Time 2 :
Demonstrations.

Leap Seconds Caveat.

The input is the argument, string or number, for JavaScript's `new
Date()` giving at least year, month, and day; allowable notations
are dependent on your browser (Date field separator /, integer seconds,
optional Offset indicator). It can be a number, but not an expression,
corresponding to the number of milliseconds since 1970-01-01 00:00:00
UTC; but *caveat* Leap Seconds.

I believe that these are strictly correct in all locations; but check. See in JavaScript Date and Time 2 : Demonstrations for running clocks and another converter, and Julian/Gregorian Calendar Date Conversion for conversions between Calendars and CMJD.

Magnitude is logarithmic; the brightest stars are about Mag 0 and the dimmest visible are about Mag 6. A difference of 5 magnitudes represents a factor of 100 in brightness, so +1 Mag is a factor of about 0.398. Avoid negative brightness. Doubling the distance reduces the magnitude by close to 1.5.

At 1 *g* in a Newtonian universe it takes a year or so to
reach the speed of light. The Solar System is only about a light-day
across; the nearest stars are several light-years away. Therefore, at
1 *g* the Newtonian approximation is good for local and
interplanetary travel; but for interstellar travel the journey is
substantially relativistic.

For Newtonian travel, mid-point turn-over in order to stop halves the distance and speed.

For telescope resolution, use Baseline for Wavelength and Distance for Aperture; watch the units. Visible light is about 600 nm wavelength; enter about 6E-7 m. Accepts expressions :-