Spatial databases

A spatial database stores information about objects in space which we can express in the form of geometry such as points, lines and polygons or geography. These are data types that are very different from the customary varchar, int, varbinary etc data types for databases. Spatial data is also queried very differently.  The open geospatial consortium defines the following query types
Spatial measurements
Spatial functions
Spatial predicates
Constructor functions
Observer functions
NoSQL systems like MongoDB and CouchDB support only a limited set.
Also the index used for spatial data types are different too. Due to the nature of the spatial data types, some patterns are very common in spatial queries and regular indexes don't support these very well. For example, how far two points differ or if a point is within a spatial area of interest require algorithms and data structures that are different from B-Trees or bitmap indexes.
For example in Microsoft SQL Server, you can do the following:
CREATE TABLE SpatialTable
( id int IDENTITY (1,1),
GeomCol1 geometry,
GeomCol2 AS GeomCol1.STAsText() );
GO

INSERT into SpatialTable (GeomCol1)
VALUES (geometry::STGeomFromText('LINESTRING (20, 180, 100, 100, 180, 180)', 0));

INSERT into SpatialTable(GeomCol1)
VALUES (geometry::STGeomFromText('POLYGON ((0, 0, 150, 0, 150, 150, 0, 150, 0, 0)', 0));

DECLARE @geom1 geometry;
DECLARE @geom2 geometry;
DECLARE @result geometry;

SELECT @geom1 = GeomCol1 FROM SpatialTable Where id = 1;
SELECT @geom2 = GeomCol2 FROM SpatialTable Where id = 2;
SELECT @result = @geom1.STIntersection(@geom2);
SELECT @result.STAsText();

Incidentally, expressing spatial data types as .NET UDTs and performing spatial operations is practical but just not deep enough for the kind of features and the size of data such as for astronomical study.