Caption: Constellation Gemini (The Twins) (zodiac symbol ♊) on a sky map of a portion of the celestial sphere.

Features:

1. Gemini is a traditional zodiac constellation and is one of the IAU-defined 88 constellations.

2. The International Astronomical Union (IAU) is the international organization of astronomers which has arrogated to itself the right to name things in the sky.

3. The IAU-defined 88 constellations are actually patches (i.e., patches of solid angle) on the celestial sphere which tile it completely without overlap.

4. All astronomical objects can be located by constellation (in the IAU-defined 88 constellations system): e.g., astronomical object X is in Gemini.

5. Gemini is crossed by the ecliptic.

6. The Sun moving eastward (which to the left on the sky map) on the ecliptic (the blue line on the sky map) is in constellation Gemini (which is NOT the same as astrological sign Gemini) in the approximate period Jun20--Jul20 (∼ 30 days) (see Wikipedia: Zodiac: Table of Dates).

7. The stars Castor (α GEM) and Pollux (β GEM) are usually NOT considered a double star since they are much farther apart in angle (∼ 5°) than the field of view (FOV) of ordinary telescopes and are easily seen as 2 stars with the naked eye.

   However, they are an obvious pair on the sky for naked-eye astronomy.

   Their angular separation is 4°30'19.53'' at some epoch, maybe the J2000 epoch (see Distance between Pollux and Castor?).

8. Castor---to naked-eye astronomy a single star---is actually a multiple-star system: i.e., a small number of stars that are gravitationally bound.

   To be specific, Castor is a sextuple consisting of visual triple (i.e., 3 apparent stars) each of which is spectroscopic binary (a binary system only identifiable as such via spectroscopy and the Doppler effect).

   The two brightest members of the visual triple constitute a double star in small telescope observation: i.e., Castor A and Castor B (see Wikipedia: Castor: Physical properties) which are separated by 4.87'' in 2013 (see Observer's Handbook, Royal Astronomical Society of Canada).

   It takes excellent seeing for near the Las Vegas Strip (∼ 4'') to resolve Castor A and Castor B.