computer science

XEphem

XEphem is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand XEphem rather than just read about it. In short: XEphem is a Motif based ephemeris and planetarium program for Unix-like operating systems developed by Elwood C. Downey.

Key takeaways

XEphem belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect XEphem to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of XEphem from memory before moving on to harder problems.

Reference excerpt

XEphem is a Motif based ephemeris and planetarium program for Unix-like operating systems developed by Elwood C. Downey.

History XEphem started as a Unix and Motif conversion of the IBM PC-based ephem. It was initially released in December 1993 with version 2.5. Its commercial edition was discontinued in 2016; the free version continued to be offered as proprietary software. In 2021, however, Downey relicensed XEphem's source code under the MIT License, raising the release version from 3.7.7 to 4.0.0 to highlight the change.

Algorithms and models XEphem uses

The VSOP87D planetary theory (full and reduced precision) for Solar System ephemeris, Approximation to DE200 for the outer planets and Pluto, and Formulae from J. Meeus (1982) for Jovian and Saturnian natural satellites Model by the Bureau des Longitudes for Martian and Uranian natural satellites and includes

About 452 million stars from both the Tycho-2 Catalogue and a magnitude limited subset of the Guide Star Catalog II, About 1 million deep sky objects mainly from a subset of HYPERLEDA, About 288,000 minor planets and comets orbital elements from the IAU Minor Planet Center and Lowell Observatory (that can be updated), and other specialized catalogs. It also include the Digital Lunar Orbiter Photographic Atlas of the Moon. XEphem is a client for Internet data sources such as the Digitized Sky Survey, Solar and Heliospheric Observatory, AAVSO light curves, and global temperature and cloud coverage. Through the Instrument Neutral Distributed Interface, XEphem can control some models of amateur telescopes, such as by Meade, Celestron, and Vixen, and auxiliary telescope components.

Catalogs While the free version of XEphem only includes a subset of the SKYMAP Master Catalog and the Messier Catalog, the internal format of the remaining catalogs can be inferred from the source code, and e.g. the internal binary Tycho-2 catalog can be generated from the original data. This is also possible for the non-stellar catalogs in the ASCII .edb format, such as for HYPERLEDA. XEphem can also read several astrometric catalogs in their original formats:

GSC 1.2 and GSC-ACT USNO A/SA 1.0/2.0 UCAC2 Numerical routines are used in PyEphem with permission of Elwood Downey.

Worked examples

Example 1 — a first encounter with XEphem

Start with the simplest possible case. Write down what XEphem claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to XEphem before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about XEphem ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of XEphem

In research: XEphem appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses XEphem in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: XEphem is common in secondary-school and first-year university syllabi. It links to neighbouring topics Free astronomy software, Planetarium software for Linux, Science software for macOS, so understanding it makes those chapters shorter.
In everyday life: Look for XEphem outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study XEphem in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what XEphem means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain XEphem out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is XEphem in simple terms?

XEphem is a Motif based ephemeris and planetarium program for Unix-like operating systems developed by Elwood C. Downey.

Why does XEphem matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study XEphem?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on XEphem.