I have some information to report but no time to report it. Mary Louise VanDyke at the Dictionary of American Hymnology sent me the following:

Thank you for your query. I can respond to part of it now; my reply may be in several installments.   

According to our files, William Horn was born in Germany May 1, 1839. He died in 1917. He came to U.S. in 1855 and settled in Wisconsin. Licensed in 1861, he rose through the ranks of ordained elder and bishop. He retired as Bishop in 1915 and died April 27, 1917.

He was the editor of various Evangelical German-lanuage publications
including EVANGELISCHES GESANGBUCH of 1877, for which he supplied a number of hymns.

Of his twenty-four hymns the most famous, according to Ellen J. Lorenz, was "Pure and free from all corruption". He also translated many English hymns into German.

Mary L. VanDyke for Dictionary of American
Hymnology,Oberlin
College Library

and Darlene Schroeder at the Mennonite Historical Museum in Kansas sent me a huge PDF image of the German text from an undated old hymnal (she guesses 1903, I think older), 2 stanzas only. Unfortunately it's all in Fraktur and will take me awhile to decipher.

So it looks like it is late 1800s or at least pre-Great-War. Still not clear whether Horn was Mennonite, or merely popular with them ("Evangelisches" sounds unmennonitisch to me, but I don't know). And his first name "William", if he was 16 when he immigrated and spent most of his life writing German I would expect his name to be Wilhelm; again, I don't know.

I agree with Wilfried that the Sun = Jesus imagery is too stock a Christian item to allow identification of the hymn with the Ambrose/Newman attribution or the Howard.

Haruo