Many others' comments focus on palindromes, suggesting cycles in the tracks' sequence.
The track, and therefore the album begins with a short trumpet vignette. In the Bible, the Book of Revelation says that the end of times - the apocalypse - would be heralded with the sounding of trumpets. This fits in with the themes and motifs of the album.
Embers observed the following quote: "From the Twins come less laborious callings and a more agreeable way of life, provided by varied song and voices of harmonious tone, slender pipes, the melodies inborn in strings and the words fitted thereto: those so endowed find even work a pleasure. They would banish the arms of war, the trumpet's call, and the gloom of old age: theirs is a life of ease and unfading youth spent in the arms of love. They also discover paths to the skies, complete a survey of the heavens with numbers and measurements, and outstrip the flight of the stars: nature yields to their genius, which it serves in all things. So many are the accomplishments of which the Twins are fruitful." - Marcus Manilius (fl. 1st century AD), a Roman poet and astrologer, said of the Gemini twins in his poem Astronomica.
Astronomica itself is a hexameter didactic poem.
The number 6 and associated symbolism (primarily, hexagons) is a recurring item in Boards of Canada's music.
Didactic refers to something that is "intended to teach, particularly in having moral instruction as an ulterior motive." - Tomorrow's Harvest could be interpreted as didactic, commentating on how our current behaviours affect ourselves, our environment and inform our future (direct instruction / teaching), but also providing an opportunity for self-reflection, reparations and a path to salvation (moral instruction - "a more agreeable way of life", "banish the arms of war", "life of ease...in the arms of love").
Specific sub-quotes can be said to refer to the Boards of Canada's themselves, imagery of flying or other representations of air, cycles, technological sophistication, destruction, life, death, apocalypse, etc. See user's post for some interpretations.
Gemini was an American spaceflight program from 1962-1966. 
Gemini is a subcritical nuclear test project ran by The National Nuclear Security Agency (NNSA). The NNSA runs the U.S. nuclear warhead program. 
Gemini (♊) is also the third astrological sign in the Zodiac. Gemini were twin brothers, and therefore, could be a symbolic reference to the Sandison brothers themselves, even though they are not identical.
Gemini can be also a direct reference to Gemini Films, a film production and distributor.  The duo already made a small reference to them, when they sampled the ending of the 1982 adult film titled "A Brief Affair"  on One Very Important Thought.