"Warning: Alien Craft Advancing!"

Platform: TI-99/4a
Genre: Sidescrolling Space Shoot 'Em Up
Developer/Publisher: Texas Instruments
Release Year: 1982

This is a fun little shmup that was, in many ways, head and shoulders above similar games of the era. Its graphics were far superior to the competition, with the spaceships and landscape well-defined and reasonably well-done; compare this to the quite blocky graphics of the Intellivision and Atari 2600. The game also made use of the TI-99/4a's optional "voice synthesizer", warning you of impending danger ("Asteroid Belt Ahead!"), complimenting your shooting or flying ("Laser On Target!"), as well as other little tidbits.

In the game you operate a green fighter ship, travelling across an unknown planet for unknown reasons. The landscape is a wasteland littered with silos, spaceships, and the occasional Texas Instruments logo. You can move the ship in the eight cardinal directions, and fire a laser beam forward. In a departure from similar games at the time, firing the laser too long will eventually cause your ship to overheat and explode (it will turn red as a warning); in addition, you have three speed settings, and a limited supply of fuel.

Each level progresses in much the same way. Alien kamikaze craft will approach you, and upon hitting the edge of the screen will scroll back around to the other edge, and stay until you destroy them. Then attack vessels will come, firing photon missiles at you at incredible speeds. This will repeat twice more, with fuel stations at certain intervals, until you reach an asteroid belt. Once you get through that, a new level will begin with much the same pattern.

Besides the repetitive nature of the game, there are a few other downsides. The attacking ships fire nearly as quickly as you do, making their missiles difficult to dodge. There is virtually no margin for error. This is also true of your fuel supply, which will usually be just about failing when you reach a fueling station. Then, too, it seems like the fueling stations were designed by sadists; they're all in cavernous, spiked tunnels that are very difficult to maneuver in.

If you own a TI-99/4a, you probably already have this game; it's one of the easiest to find, I believe. If you don't, there are several emulators around, but it's considerably harder to find ROMs for TI-99/4a games than many other systems, despite the fact that it had quite a large fan base even long after Texas Instruments abandoned it. Even so, however, I'm not sure how much fun it will be to someone who lacks nostalgia for it. Still, it's worth checking out if you enjoy shmups at all; in some ways, it's more complex than some of the shmups being made today.

Parsec stands for Parallax Arc Second, and is a measure of astronomical distances approximately equal to 3.26163626 light years. The parsec is a useful unit of measurement because it directly translates a measurable stellar phenomenon into a distance measurement.

Parallax

Parallax is the name given to the apparent movement of an object against its background as the observer's viewing angle changes. As the Earth orbits the Sun, its viewing angle changes enough to cause measurable parallax with stars up to 1000 parsecs away from space based telescopes. Earth based telescopes are restricted by a reduction in resolution caused by the atmosphere, and can only measure parallax in stars up to 100 parsecs distant.

Parallax must be measured against an unchanging background, and because stars greater than 1000 parsecs distant exhibit no measurable parallax, these background stars are used for this reference. The parallax of the stars is measured in arcseconds. One caveat is that the stars appear to move across the sky over long periods of time, and this proper motion must be taken into account when measuring parallax.

Arcsecond

Objects and distances in the sky can not be measured in centimeters or inches. Instead, distances in the sky are measured in terms of how much of the sky's arc they cover. For most objects, this is on the order of arcseconds — 1/3600 of a degree. The Sun, for example is about 1,900 arcseconds in diameter, and the Moon is only slightly smaller at 1,800 arcseconds. One arcsecond is just over one mile on the Moon as seen from Earth.

Parsec

The parsec is defined as the distance to a star with a parallax of one arcsecond when the viewing position changes by one astronomical unit (AU). This means that the parallax should be measured over one quarter of the Earth's orbit so that a right triangle is formed between the Sun, the Earth, and the measured star. This is one half of the maximum possible parallax observable over a full six month period.

   g
 b r s                                                           Earth
 a o t
 c u a
 k n r          Star∠                                    Earth    Sun
   d s
Distances not to scale, ∠ represents the parallax angle

Because the distance from the Earth to the Star is on the order of light years, and the distance from the Earth to the Sun is on the order of light minutes, this is a very narrow triangle (with an angle measured in arcseconds), and the small angle approximation applies with virtually no reduction in accuracy. This simplifies the math to:

     1
d = ---  where "d" is in parsecs and "p" is in arcseconds.
     p

For example, the closest star to the Sun, Proxima Centauri, has a measurable parallax of about 0.77 arcseconds, so it is about 1.3 parsecs, or 4.2 light years, away.

Hipparchus

The idea of stellar parallax was first proposed by the Greek astronomer Hipparchus (190–120 BC), most famous for developing the apparent magnitude classification for the brightness of stars. The heliocentric and geocentric theories of the solar system were being debated at the time, and Hipparchus realized that if the Earth did indeed orbit around the Sun, there should be observable parallax in the stars, assuming the stars are not infinitely far away. Unfortunately, without the aid of telescopes, Hipparchus was not able to measure any parallax in the stars and concluded the geocentric model was correct. The unaided human eye has a resolution of about 60 arcseconds (opinions vary widely), far too large to detect stellar parallax. By comparison, the Hubble Space Telescope has a resolution of about 0.05 arcseconds (20 parsecs, or 65 light years) (to put that in perspective, there are about 1,000 stars within 50 light years, 133 of which are visible to the naked eye).

Friedrich Wilhelm Bessel

It was not until 1838 that the German astronomer Friedrich Wilhelm Bessel was the first person to be able to measure stellar parallax, using a Fraunhofer heliometer. He chose the star 61 Cygni to measure, reasoning that a star with a large proper motion would be relatively close to the sun, and therefore have a large, measurable parallax — he measured its parallax as 0.314 arcseconds, placing it at 3.18 parsecs, or 10.3 light years (its actual parallax, measured with modern equipment, is 0.292 arcseconds, 3.42 parsecs, 11.2 light years).

Hipparcos

In honor of Hipparchus' many achievements in astronomy, the High Precision Parallax Collecting Satellite (Hipparcos) was named for him (sort of). The Hipparcos satellite ran from August 1989–August 1993 and, among other things, observed parallaxes for about 2.5 million stars, out to a distance of about 1000 parsecs.

Sources:
http://wind.cc.whecn.edu/~marquard/astronomy/parameters.htm
http://www.astro.washington.edu/larson/Astro101/LecturesFraknoi/astro101s07.html
http://brahms.phy.vanderbilt.edu/~rknop/classes/a250/fall2002/wahlig/
http://www-history.mcs.st-and.ac.uk/~history/Mathematicians/Bessel.html

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