The Toronto Magnetic and Meteorological Observatory, formally the Louis B. Stewart Observatory, is a historical observatory located on the St. George campus of the University of Toronto, in Toronto, Ontario, Canada. The original log building was constructed in 1840 as part of a worldwide research project run by Edward Sabine to determine the cause of fluctuations in magnetic declination. Measurements from the Toronto site demonstrated that sunspots were responsible for this effect on Earth's magnetic field. When this project concluded in 1853, the observatory was rebuilt in stone by the Canadian government and served as the country's primary meteorological station and official timekeeper for over fifty years. The observatory is considered the birthplace of Canadian astronomy.

Compasses tended to "wander" from north when measurements were taken at different locations or even at a single location over a period of time. The astronomer Edmund Halley noted this and the problems it would cause for navigation in 1701. It was also believed that whatever was causing this effect might be causing changes in the weather, and that studying magnetic variations might lead to better weather prediction.

In 1833 the British Association for the Advancement of Science commissioned a series of magnetic measurements across the United Kingdom. Under the direction of Major Edward Sabine of the Royal Artillery, a multi-year measuring project began, with the results to be published in 1838. As the measurements were being made a number of proposals were put forth to expand the program worldwide. In 1836 the German explorer and naturalist Alexander von Humboldt wrote to Prince Augustus Frederick, Duke of Sussex, then President of the Royal Society, stating that a formal program was important to a nation with dominions spread across the globe. At the seventh meeting of the British Association in Liverpool in 1837, Sabine declared that "the magnetism of the earth cannot be counted less than one of the most important branches of the physical history of the planet we inhabit" and mapping its variations would be "regarded by our contemporaries and by posterity as a fitting enterprise of a maritime people; and a worthy achievement of a nation which has ever sought to rank foremost in every arduous undertaking".

In 1837, the British Government funded the installation of a magnetic observatory at Greenwich. The Association continued to press for the construction of similar observatories around the world, and in 1838 their suggestions were accepted by the Government and funds were provided. In 1839 the British Government and the Royal Society prepared four expeditions to build magnetic observation stations in Cape Town; St. Helena; Hobart, Tasmania and (eventually) Toronto, Ontario. Teams of Royal Artillery officers were sent out to take the measurements. The team assigned to Canada originally planned to build their observatory on Saint Helen's Island off Montreal, but the local rocks proved to have a high magnetic influence, and the decision was made to move to Toronto instead. The team arrived in 1839, and set up camp at Fort York in a disused barracks while construction started on new buildings. The observatory was given 10 acres (4.0 ha) of land to the west of King's College; the Ontario Legislature now occupies the area on which the college was located.

The observatory, officially "Her Majesty's Magnetical and Meteorological Observatory at Toronto", was completed the following year. It consisted of two log buildings, one for the magnetic instruments and the other a smaller semi-buried building nearby for "experimental determinations". The north end of the main building was connected to a small conical dome which contained a theodolite used to make astronomical measurements for the accurate determination of the local time. The buildings were constructed with as little metal as possible; when metal was required, non-magnetic materials such as brass or copper were used. A small barracks was built nearby to house the crew.

Using the measurements from the Toronto and Hobart sites, Sabine noticed both short-term fluctuations in magnetic declination over a period of hours, and longer-term variations over months. He quickly concluded that the short term variations were due to the day/night cycle, while the longer term ones were due to the number of visible sunspots. He published two introductory papers on the topic in the Philosophical Transactions of the Royal Society. The first, in 1851, was a collection of early measurements; the second in 1852 correlated with Heinrich Schwabe's sunspot measurements, which had been made widely available in Alexander von Humboldt's Cosmos, also published in 1851. With further data collected from the Toronto site, Sabine was able to demonstrate conclusively that the eleven-year sunspot cycle caused a similarly periodic variation in the Earth's magnetic field. He presented a third and conclusive paper on the topic in 1856, "On Periodical Laws Discoverable in the Mean Effects of the Larger Magnetic Disturbances", in which he singled out the Toronto site for particular praise.

Sir John Henry Lefroy, a pioneer in the study of terrestrial magnetism served as director of the magnetic observatory from 1842 to 1853; In 1960, the Ontario Heritage Foundation, Ministry of Citizenship and Culture erected a Provincial Military Plaque in his honour on the University of Toronto campus.

In 1853 the Royal Society's project was concluded, and the observatory was set to be abandoned. After a lengthy debate, the fledgling colonial government decided to take over its operation. Rather than disappearing like its three counterparts, the Toronto observatory was upgraded, and its mission was expanded as it became a meteorological station (see Meteorological Service of Canada) under the direction of the Ministry of Marine and Fisheries. During the expansion, the original buildings were replaced with a permanent structure.