1987 AD., Portugal, Republic, Diogo Cão commemorative, Lisbon mint, 100 Escudos, KM 641.

Portugal, Republic, Golden Age of Portuguese Discoveries series - explorer Diogo Cão commemorative, engraver: Paulo Guilherme d’Eça Leal, Imprensa Nacional - Casa da Moeda, Portugal, 1987 AD.,
100 Escudos (ø 34 mm / 16,24 g), copper-nickel (75% copper, 25% nickel), 16,50 g theor. mint weight, mintage 1.000.000 , axis coin alignment ↑↓ (180°), reeded edge,
Obv.: REPUBLICA PORTUGUESA / 1987 / 100 ESC. / 22 13 6 , Portuguese shield of arms to upper right of circular nautical astrolabe design, value below.
Rev.: 1486 - D'EÇA - incm / DIOGO CÃO / 22 13 6 , a rose-of-the-winds, radiating lines of direction, compass within center of a medieval sailboat and map of Europe and Africa to right.
KM 641 .

Year / Mint Mark / Mintage
1987 / incm / 1,000,000

Diogo Cão was a Portuguese explorer and one of the most notable navigators of the Age of Discovery. He made two voyages sailing along the west coast of Africa in the 1480s, exploring the Congo River and the coasts of the present-day Angola and Namibia. Sent by King John II, he made two voyages of discovery on the southwest African coast between 1482 and 1486. He arrived at the mouth of Zaire and advanced through the interior of the river, leaving an inscription confirming his arrival at the falls of Ielala, near Matadi. He established his first relations with the Kingdom of Congo. In 1485 he arrived at Cabo da Cruz (now Namibia). He introduced the use of stone monuments, instead of wooden crosses, to mark the Portuguese presence in the discovered areas.
More on https://en.wikipedia.org/wiki/Diogo_Cão

An astrolabe (Greek: ἀστρολάβος astrolabos; Arabic: ٱلأَسْطُرلاب‎ al-Asturlāb; Persian: اِستاره یاب‎ Astaara yab) is an elaborate inclinometer, historically used by astronomers and navigators to measure the altitude above the horizon of a celestial body, day or night. It can be used to identify stars or planets, to determine local latitude given local time (and vice versa), to survey, or to triangulate. It was used in classical antiquity, the Islamic Golden Age, the European Middle Ages and the Age of Discovery for all these purposes.
The astrolabe's importance not only comes from the early development of astronomy, but is also effective for determining latitude on land or calm seas. Although it is less reliable on the heaving deck of a ship in rough seas, the mariner's astrolabe was developed to solve that problem.
More on https://en.wikipedia.org/wiki/Astrolabe