  We review the relatively new, internet-enabled, and rapidly-evolving field of
citizen science, focusing on research projects in stellar, extragalactic and
solar system astronomy that have benefited from the participation of members of
the public, often in large numbers. We find these volunteers making
contributions to astronomy in a variety of ways: making and analyzing new
observations, visually classifying features in images and light curves,
exploring models constrained by astronomical datasets, and initiating new
scientific enquiries. The most productive citizen astronomy projects involve
close collaboration between the professionals and amateurs involved, and occupy
scientific niches not easily filled by great observatories or machine learning
methods: citizen astronomers are most strongly motivated by being of service to
science. In the coming years we expect participation and productivity in
citizen astronomy to increase, as survey datasets get larger and citizen
science platforms become more efficient. Opportunities include engaging the
public in ever more advanced analyses, and facilitating citizen-led enquiry by
designing professional user interfaces and analysis tools with citizens in
mind.
