- How to find and observe the Double Cluster (TOTS#5) (eyesonthesky.com)
NGC 869 and NGC 884 are two open cluster within our own Milky Way galaxy that are very close to each other, creating a “dual” cluster for telescope observers. The two clusters are each about 7,500 light years from Earth, so the photons from it reaching your eye in a telescope left that cluster about the time copper was being used in the Middle East. The estimated age is about 12.8 million years, making it one of the younger star clusters in our galaxy. Here is a detailed guide on how to locate the Double Cluster.
- Existential quantification (Wikipedia)
In predicate logic, an existential quantification is a type of quantifier, a logical constant which is interpreted as “there exists”, “there is at least one”, or “for some”. It is usually denoted by the logical operator symbol ∃, which, when used together with a predicate variable, is called an existential quantifier ("∃x" or “∃(x)” or “(∃x)”). Existential quantification is distinct from universal quantification (“for all”), which asserts that the property or relation holds for all members of the domain. Some sources use the term existentialization to refer to existential quantification.