What is collected? An excellent aid regarding the U.S. Census information from 1790 through 1930 is offered by Genealogy.com

Union County Illinois Federal Census Records include: 1820, 1830, 1840, 1850, 1860, 1870, 1880, 1900, 1910, 1920, 1930 and 1940.

1890 Union County Illinois Federal Census Records (including the Veterans and Widows census) were completely destroyed by a 1921 warehouse fire.

Census records for Moses and Amy Knupp  185018601865, 1870188019001920  (contributed 18 Feb 2018 by Melissa McCord)

Searchable collections at FamilySearch:

1818 Illinois State Census is available free online at Access Genealogy
    Union County -
        A-D Surnames
        E-K Surnames
        L-R Surnames
        S-Z Surnames

All of the census records are available free online at Archive.org. You can read and even download the massive files.

Templates in Time