Both equinoxes ... September 21 and March 21.
Answer:
The problem occurs with all spherical mirrors.
Spherical mirrors are practical up to about inches in diameter.
Reflecting telescopes use spherical mirrors for apertures up to about 4 ".
Larger aperture telescopes use parabolic mirrors to obtain sharp focus.
Answer:Three elements make up over 99.9 percent of the composition of dry air: these are nitrogen, oxygen, and argon.
Explanation:
Answers:
a) 
b) derived
Explanation:
Density 
is a characteristic property of substances and materials and is defined as the relationship between the mass 
and volume 
of a specific substance or material, as shown below:
This means density is inversely proportional to volume.
In addition, density is a derived unit, because is based on two basic units of the International Sistem of Units: Mass (kilograms) and Volume (cubic meters)
